Can't remember the exact size but John Lewis sell their own brand sheets which are wayyy cheaper than the chicco ones and they fitted perfectly. I think it was in the Moses basket sheet range, they had smaller and larger ones. If you get the dimensions of the chicco mattress and look at John Lewis's website it used to have the sheet dimensions in there.
Sorry not massively helpful I know, mine are all stored away right now!!!
I just used the ones that I'd bought the baby didn't wriggle enough to even disturb them even when bigger. The mattress is very snug any way they were fitted but too big I just tucked them in and under it always looked near and snuggly.