I'm sure you could use anything from 1/16" up to 1/2" wide stuff, but the pattern needs to get smaller (tighter) as the size of the strands gets narrower in order to have the same strength and an even look. I suppose it depends on what you like the looks of best. Personally, the nicest set I've owned were back in the short period when Vermont Tubbs owned a little struggling company called Mad River Canoe. The lacing was about 1/4" wide and the biggest holes were maybe 3/4" across or so. It's also a good size for general durability and difficult to accidentally snag or punch through.
Tips:
Buy full-grain rawhide if you can find it. The split stuff usually means that somebody else got the good part and it isn't as tough or as resistant to stretching.
Keep your splices off to the sides, where you won't be sitting on them or snagging them with your legs if kneeling because once dry, the edges are sharp.
There is a right side and a wrong side to the lacing. The right side (skin side) is smoother and will dry in a slightly convex curve (in cross-section). You want the right side up as you weave so that it's the side up against your posterior.
After weaving and drying, but before varnishing, go over the seat with a bare hand and feel for sharp edges on the corners of the strands. Hit any you find with fine sandpaper to ease the corners a bit (but don't scrape-up the top, flat sides). If left raw, these edges are pretty abrasive. I once wore holes in the seat of a fairly new set of jeans during a ten-day Quetico trip and so did my bow man.
Above all, when getting ready to weave do not soak the rawhide for more than about 24 hours. Normally, it's just a bit slimy to work with, but if you soak it too long it smells EXTREMELY bad!
This photo shows a pattern about the size of the ones I used on seats.