Any suggestions on possible replacement speakers that would imrove the performance of the standard head unit in a 2003 EX?

Crutchfield (http://www.crutchfield.com/cgi-bin/S-w2G7q7tC4nv/autoinfo/autoinfo.asp?s=0&cc=01&vehicleid=0) is probably the best place for size and fit information for any vehicle.

go to Crutchfield to see what fits and a brand you like. Then go to www.pricescan.com and find it cheaper. ;)