"Is my David copperfield book rare?"
Probably not. However, you could check out sites such as E-Bay and Amazon to see if that edition turns up. And you could also try Googling.
"It was published by modern library in new York it has illustrations by " phiz "..."
That's a publisher that tends to do big print runs of relatively cheap editions. Such is not the stuff of rarity. Phiz was an illustrator who worked with Charles Dickens when he was still alive and writing. Those illustrations come from much older English editions, and have been re-used many, many or ever so many times.
I can't remember offhand when Copperfield first appeared, but it must've been around the 1830s. Even should that copy of yours be from the 1920s, then the book had been coninuously in print for much of a century by then. Now, should you happen upon the first edition (which would probably be a three volume set, as was common in Britain in the first half of the nineteenth century largely because of the needs of subscription lending libraries, who used to be about half of those editions), that would be rare. (First print runs were then typically in hundreds or, exceptionally, one or two thousands even for ever so well-established authors. They were seriously expensive to buy!)
I'm a secondhand book-handler.