Sooo... Thinking of trying a pipe.

OK, a disclaimer--much will depend on your preferences. You won't know what those are until you experiment. For instance, I cannot enjoy most aromatics, they taste harsh to me, the flavors added to the tobacco taste artificial, and often the taste of the tobacco itself is overwhelmed; the topping and casing is usually applied with a carrier of propylene glycol (PG) which also keeps the tobacco too moist and steamy/hot for my taste. But I also dislike latakia, used in most English and Balkan blends. You may just adore these however.

-Why are stem length different?
That's an aesthetic privilege of the pipe carver and evolution of the shapes over the last few centuries combined with the taste of the buyer. There are 'nose warmers' with really short shanks and stems, Canadians with really long shanks and short stems, etc. Part of the choice will come down to how you smoke, do you hold the pipe in your hand and take occasional puffs or clench it in your teeth. A long straight pipe has more leverage and if heavy will be unpleasant for clenching. A bent pipe overcomes some of the weight/leverage by offsetting the balance closer to your mouth and is often preferred by clenchers. A good first pipe would be a modestly sized billiard; many claim they are the least fussy.

Alright, this could turn into a dissertation, so, briefly:

-What is the point of different bowl sizes?
Some tobaccos or cuts of tobacco taste better in a short, wide bowl. Others in a medium, others in a tall, narrow bowl. The width and height of the bowl creates different, evolving, flavors, burn hotter or cooler, incorporate more or less air per puff. When one finds the right combination it can be magic. Bowl wall thickness plays a factor in how hot or cool the tobacco burns, but so does smoking style, blend, and cut.

-How do you clean a pipe?

Run a pipe cleaner through it while smoking if it starts to gurgle, otherwise do so after you smoke. Don't take the stem off until at least 1/2 hour or you will eventually risk a loose stem. After a one or two bowls I put the pipe in the back of the rotation, giving it at least a couple of days to dry out. Cake will slowly build up in the bowl sides. The standard advise is to ream it back to about the width of a dime. I use a pen knife but many invest in a reamer. For a start I would suggest some pipe cleaners and a 'Czech pipe tool' which contains a tamper, a poker for loosening a too-tightly packed bowl, and a spoon for scooping out the 'dottle' at the bottom of the bowl.

-Difference between a corncob, meerschaum , or briar pipe?

A cob is the least expensive, cool-smoking, quick-drying, inexpensive. I love mine. Meers impart the least flavor to the tobacco, think 'neutral', and they are fun to watch color over time. Just don't drop them. Briars are very resistant to burning unless you smoke like a furnace, well cared for they will last a lifetime. Some are utilitarian, others are amazing sculptures with incredible grain and cost quite a lot, think 1-2 thousand dollars. I'd suggest a cob and a briar or two in the $75 to under $300 price range. 

-What are aromatics? Why are they harder for beginners to smoke?

see 'disclaimer' above^

-What pipe tools do you need and what are they used for?

see -How do you clean a pipe?^

-Any suggestions for blends?

Depends on taste. I'd try an aro like CAO/Dan Sweet Vanilla Honeydew or Blue Note, a VA/Per like Gawith Louisiana Flake, or C&D Night Train, Escudo is very tasty, Peterson's University Flake is a favorite and has an extremely light berry casing, Three Nuns is nice, some of the Dunhill English blends like Early Morning Pipe. Experiment....
pipesandcigars.com/pipe-tobacco/tin-samplers/

-Are there mild and full body blends?
Yep. Blends that contain a lot of Cavendish or white burley tend to be mild, ropes tend to be very strong nic-wise, VA's tend toward the naturally sweet, fire-cured Kentucky strong and smoky, straight burley in a shag cut will put hair on your chest--Five Brothers comes to mind.

-Can you mix blends to create your own?

Sure, or try these:
pipesandcigars.com/pipe-tobacco/blending-tobaccos/

-Is pipe tobacco short or long filler? 

While you can certainly buy whole leaf, all pipe tobacco needs to be cut in some way. Flakes are pressed in bricks and sliced, ready-rubbed are flakes that have been rubbed out, and many blends are ribbon or shag cut.

-I heard a rumor that you need to twists the tobacco when packing it in a bowl. True?
Most smokers will fold,  twist, or rub out flakes to get them to burn evenly. Packing a pipe is an acquired skill and depends on the consistency of the blend. Moist blends often benefit from some drying time, pliable but closer to dry but short of crispy, much like cigars, speaking of cigars--C&D Habana Daydream and GL Pease Robusto are blends that incorporate cigar leaf, both good.

-How do you pack a bowl?
With ribbon cut types, bottom third very gently, middle third light pressure, top third heavier pressure is one way. Cube cut burleys are usually gravity fed, flakes are either rubbed out or folded. The compressed cube cuts and flakes will expand while burning so best to not pack too tightly.

-What kind of lighter. Soft flame or jet? -Do you char the tobacco as you do a cigar?

Never a jet lighter, you'll for sure burn the rim of the pipe. Soft flame or wooden matches. Do a false light. Stop for a minute, tamp, and re-light. Expect to light several times during the course of the bowl. Slow puffing is the best way to bring out the true flavors of the tobacco, fast puffing creates too much heat.

-How should pipe tobacco be stored? In a humidor?

Air tight container. 

PM me if you have other questions, would like some samples, or if the couple of links in this essay get expunged.