I've used 0.6mm lead-free solder before for perf board projects, and I found it worked pretty well for most components. It's not too thin, but if you're soldering things like bigger caps or connectors, a thicker solder, like 0.8mm, might help fill those joints better. Personally, I prefer leaded solder because it flows nicely and makes for cleaner joints, but I get that lead-free is a good choice too.