Sprokets,
I am also a big guy at 6' 5 and 255 wth size 15. I think you will be able to find a few spd's in that size range just ask the lbs's for recommendations. Currently I haven't found a company that carries my range.

Cadence is very important for both your knees and staying upright. If you peddle to slow you will tip. It runs along the same line as football. Keep your feet moving and you will keep moving. Move the feet slowly and better hope you aren't near a cliff.