Having done this job I'm a bit annoyed I cannot remember the size, but then it was about 10 years ago! The simplest way to do it is to keep trying drill bits in the hole until you find the right fit, then go for pop rivets of the same size. They don't have to be exactly right although quite a bit out won't work either. If you search using "pop rivet sizing" you'll find plenty of answers. It really isn't hard.