One Arduino can not handle 1 web cam.
Define "handle". USB Video class supports variety of formats and frame sizes (and still pictures of these many sizes, too). It is possible, for example, to stream a video from a webcam to nokia 6610 display (or from 4 or 127 webcams, for this matter) - I've done it on old Atmega-168 based Arduino with not much difficulty; for any size data, you need 2 bytes of memory to pass a pointer to function which reads a byte from MAX3421E buffer.