Crusiers ROCK!!!
Most smaller sized double bits, 8 to 8.5 length with blades around 3.5 to 4" edges are called cruisers. They typically are around 28" in overall length. They are light to pack, strong and do have enough length & weight to cut well for their size.
Many people have a shaving sharp edge and a duller edge for grubbing around. Easy to keep tract of if you keep the makers mark/logo (if there is one) toward you for the sharp edge.
And
CRUISERS ROCK!!! I have a few of them, I'll have to post a picture soon.
A shorter double bit, sometimes referred to as a saddle axe are not very common for a quality piece of steel.
If you are set on a 20" overall length, you will most likely be looking at a single bit axe.
But....it just might be interesting to cut a 28" cruiser down to 24". Who knows it just might work for you. But you will not have any purchase on the end of that handle.
I'm sure someone with some more experience will jump in here.
Your question is rather broad, books have been written on a complete answer for you.
If it's any help. my first axe was a 17" camp style axe (single bit), most people start off with that size as a 1st axe/ hatchet. You can watch out for them at garage sales.
Council Tool makes a decent American made axe. they have a Dayton style head, 14" Hunters axe for $32.
http://www.counciltool.com/product.asp?pg=product&item=125HU
A 20" Miners axe for $47
http://www.counciltool.com/product.asp?pg=product&item=35MD20
A 28" Boys style axe for $41 that has had several good reviews on this forum.
http://www.counciltool.com/product.asp?pg=product&item=22BR
They also have several sizes in a heavier Jersey style axe head.
http://www.counciltool.com/DisplayCategories.asp?pg=displaycategories&category=13
You'll have to decide what you want to do with your axe first, then begin to narrow down the ones that will fit that purpose.
My regards, Tom