A single Magic card is 2.5 inches wide and 3.5 inches high. Realistically speaking, you should allow ~3 inches of width and ~4 inches of height per-card. If I were making a custom mat, I'd go for 16 inches high (enough space to comfortably allow 4 rows of cards) by 24 inches wide (2' is the minimum space most places I've gamed have per-player, and this would allow for eight cards side-by-side, comfortably). Assuming your library and graveyard take up two cards' worth of footprint, a mat that size would allow for 30 unique cards laid out. Stacked cards - like lands, or enchanted/equipped permanents - increase the count. In either case, it's representative of probably the most space you'll possibly need for your own side of the field in even the craziest EDH 8-player chaos match.