M5 bolts work with most stems. Measure your bolts and choose the closest length.
A threadless star nut/top cap assembly uses M6.
M8 bolts work on Phil Wood & Paul hub axles but you might need to cut them down to the right length depending on the thickness of your dropouts.
Some common seatpost size examples: Thomson uses two M5 x 40mm, Velo Orange Grand Cru uses two M6 x 40mm, Nitto S83 and NJSP72 use two M7 x 40mm, Nitto S65, Dynamic 626, Miche Supertype and Soma Layback use one M8 x 40mm.
Not sure which size or length you need? Contact us and we'll help.