The tilter bolt is 1 1/4" and the rail bolt is 1 1/2". You are looking for Carriage bolts. The head size is just under 3/4". This is not critical but just for reference.

I do not recommend getting Stainless Steel replacements. Many good hardware stores sell chrome plated hardware. Mixing Stainless with non stainless fasteners can lead to galling of the threads.

Why are you replacing them? Is there some damage?

The single most important thing you can do to make your rail work right is to make sure the threads are clean and greased with a general purpose grease or white lithium grease. If the threads aren't lubricated, you just can't get them tight enough and will start galling the threads before getting the rail to tighten up.

Unless your bolts threads are damaged or the bolts are bent, then cleaning and lubing them should fix any problem you are having.