I made my own board, its quite easy (believe me, I'm no handyman!).
Mine is about 400x700mm made of 16mm MDF wood (I think you might call it something different in the US or UK). It has black carpet on top and maroon vinyl on the bottom. It also has aluminium channel strip around the edge and two metal handles each end (kitchen cupboard type).
I bought some velcro dots first and went to an Outdoor/Rubber supplier and tested the dots on different carpet, looking for the most sticky. The hardest part was cutting the alum channel at 45deg for the corner joins. I only had a hacksaw so its a bit dodgy, later I found you can get it custom cut when you buy it so I'd do that. It was about $40-50 total for all the materials. I'll try to post a pic if your interested.
For powersupply I just velcroed a 4 outlet board (with trip switch) to it in the corner (although I recently saw a board with one down the centre to conserve space). It has a generic DC wall wart with daisy chain, an AC wall wart for whammy and I also plug my midi floorboard into it.
As for what size you need just get some masking tape and make the shape on your floor trying to fit all the pedals you want on the board in it. Hope all that helps.
