Typically, slimline pencil kits use 0.5mm lead. I'm not, however, thrilled with the quality of the mechanism. I've made and used several - and they've all failed. Pentel makes the best mechanical pencil mechanism (in my opinion), but if you don't like the style, you can convert many Parker-style pens to pencils by using the Schmidt pencil mechanism. It is available in 0.5mm. For some kits, such as the "Perfect-Fit", the Schmidt pencil is a drop-in replacement. For others, a nib modification is necessary.
Regards,
Eric