Ipe likes a narrow, deepish cross section. One inch wide will do the trick. It really depends on the style and length of bow you want to build. I made a hickory backed ipe flatbow a few years ago(my first try with ipe). I think it was 64" long, 55#@26", 1 3/4 wide at the fades and the 1/8" hickory backing was thicker than the ipe belly. The bow shot well. Another try was 1" wide x 66" long, hickory backed ipe with a straight taper to 3/8" tips.I never got to weigh this one because of a severe fret just above the handle. I was trying to get it to bend right at the handle and strained it too much, I guess...anyway it would have been a real screamer.
Like osage, it doesn't take much ipe to make a hunting weight bow. Pat