if its a 5 lug pattern, 5x4.5 is by far the most common trailer wheel pattern, but 5x4.75 and 5x5 also exist, so its only a guess as to what you really have without measuring it...
but what Ive seen in the past, if it is a 5 lug pattern and NOT 6, my bet would definitely be on the 5x4.5..... 6 lug rims are also very common on trailers..