If you want the float glass for plane fettling, you don't want an off-cut! You'd be looking at a piece around twice the length of the longest plane you plan to fettle. This is assuming, of course, that you include sole-flattening in the description of fettling.
The bit I have, which I've snugged into a torsion box constructed frame, is about 2' long, bout 8" wide, I think.
Any old glazier will do you some though - 6mm thick should do you.
[edit - see
this wreck post for a semi-jokey, semi-serious insight into fettling...about 3/4s of the way down, you'll actually see Spokeshave recommends 3 times the length of longest plane]