As big as you can afford. Just know that good, used anvils are not cheap. Brand new ones are even more expensive(understandably). You can expect to pay $3-4 per lb for a decent used anvil. Mine isn't heavy enough, probably. I got a 127 lb Swedish Kohlsaw(sp?)anvil off Ebay a few years ago. Cost me $380 and a 3 hour drive to pick it up. I considered it a bargain then, and still do. It was used slightly, by a Farrier and he used the horn mainly. The flat face is nearly new.