Glenn,
I'd suggest you pick up a good reloading manual, such as Lyman's 49th Edition. It will have all the information you need on the tools and procedures to disassemble and reassemble rifle cartridges. It describes what each die does, and how to adjust them. The manual is also a wealth of information about the component sizes and cartridge dimensions.
If you are using the "Lee Loader", which uses a hammer to operate, I suggest you upgrade. A Lee "Reloader" press is inexpensive and about the minimum you need to assemble your own ammo. For each caliber, you'll need a set of dies and a shellholder. Lee RGB dies are bare. Lee Pacesetter dies come with a powder dipper and shellholder. Each sizing die has a decapping pin (for depriming), but I also like to use the Lee "Universal Decapping Die" to deprime shellcases without resizing.
A slimline nib should be a pretty close match for a resized 30-06Spg or 308Win case. A euro nib is too big. Some brands of brass are thicker and will have larger case mouths after resizing. Military brass, in particular, will tend to have thicker walls. If you still wind up with narrow necks, you can get larger diameter expander balls.
Once you start making your own cartridges, however, I suspect you'll switch from kit nibs to real bullets. Then you won't need to worry about matching the kit diameter.
Regards,
Eric