.5 and once a week at the start, then after a few months you can use up to 2 times a week. If using minoxidil, for first few months do not use minoxidil right after rolling, wait at least 12 hours, and probably 24 .
Most people use a 1.5mm or higher once a week. Smaller rollers seem to be effective when used daily. Given the choice between those two sizes I would use the .5mm one daily. It's perfectly alright to use minoxidil immediately afterwards. It aids in the absorption. It stings the first few times, but then you get used to it.