Great question! I would say that measuring your hand with the liner glove on would be the best way to make sure you don't end up with a system that is too tight. This is particularly important if you're crossing brands to build your system, such as an REI liner glove with a Burton mitten, as an example. Generally speaking, brands will run fairly consistently within their product lines (a large in one REI glove will fit similarly to a large in another REI glove style), however, there can be variability between brands.
One other thought is to think about the proportion of your hands. That is to say the length of your fingers and the width of your palms. As an example, I have really big palms (wide) and relatively short fingers. So I have to make sure that I fit my palm first, because if that part of the glove is too tight, my circulation is affected and I will be cold. That means that often times the fingers in my gloves are just a bit too long, but I've learned that is just a thing I will need to deal with in order to have warm hands in the winter. That also means that when I find a great fitting pair of gloves that work for me I buy two pairs because I'm not sure I'll be able to find them again when the first pair wears out!
Hopefully this helps, thanks!