I am considering creating a large LAN (possibly greater than 255 computers) using home wireless routers such as Linksys WRT54G as switches (DHCP off).
What, roughly, is the size or limit of the MAC address table for home wireless routers?
Typical chips in SOHO switches and routers like the Broadcom BCM5325 or the ADMtek ADM6996L are able to handle up to 2000 MAC addresses according to their data sheets. Exactly these chips are also found in the different hardware revisions of the WRT54G. So you will not face any problem from this limit.
As already mentioned by Hennes it is still not recommended to connect such a large number of clients by cascading 5-port switches/routers. At least unless you are not building a wireless communitiy network where the WiFi interfaces could be useful.
For a normal network it wold be much cheaper to buy some 48- or 24-port switches. If there are a lot of small groups with bigger distances between them, you might also want to use more switches with less ports to save costs/space on cables but I doubt going down to many 5-port switches would be useful in nearly all situations.
Most of these routers run a pretty modern real-time operating system capable of remembering a large number hosts, probably only limited by the memory allocated to the ARP table.
However, the switch functionality is normally provided by hardware-based switch chips with very limited control memory for mapping tables. The actual number varies between model and versions, so you have to look them up individually, but in general you're looking at about tens to hundereds of hosts per port before the switch becomes a hub. Even then, your network is still going to work, even though not very efficiently.
I suggest you fork some money and buy some proper switches...