Chryslers Hemis from '51-58 were physically the largest being longer and wider by over an inch than the '52-57 DeSoto Hemis which were physically longer and wider by over an inch than the '53-57 Dodge Hemis which I would consider "baby" since they started at 241" and went to 325". The Chryslers started at 331" and went to 392".
This is a really vague question- no offense- but there are a ton of Hemi threads here, so the search function is your first line of offense. Failing that, you might look for this book: http://www.amazon.com/Complete-Chrysler-Hemi-Engine-Manual/dp/1878772015/ref=ntt_at_ep_dpt_1 It's still in print, so any bookstore can order it, or, try Motorbooks.com.
Our local salvage yard, Yarbrough's, just pulled in an early '50s Dodge 1 1/2 ton flatbed with a hemi in it. No idea what kind of shape the engine is in internally, it's rusty and crusty externally, but that doesn't necessarily mean anything.
Google early Dodge Hemis..Lots and lots of info..I have one in my 1935 Ford Sedan..No sure of the HP, mild build and it goes fast enough for me.. Duane.