Well, the B29 had a winspan of 141'3" and a length of 99'
So,
141'3" = 1695'"
99' = 1188"
@ 1/48 scale it would be:
1695/48 = 35.31" wingspan
1188/48 = 24.75" long
Basically, about 3' wingspan and 2' long. A decent size piece of plastic whichever way you look at it.
*edited spelling errors! Oops!*