The size of a viewfinder is you see it through the standard eye-piece is magnification divided by the crop-factor. So a 0.9X on a 1.5X crop camera has the same size as a 0.6X on a full-frame.
This gives a very good approximation of the comfort of using a viewfinder. However, a better measure is how big are details shown through the viewfinder. This correlates to your ability to judge focus and fine details. For example, for a group portrait, if you see details better you will be able to notice more easily when people are blinking or not looking the right way.
In order to determine the size of details shown through the viewfinder, coverage needs to be included in the calculation. The simple reason is that if you see less in a fixed sized window, then what you see must be bigger and vice-versa.
This is what I explained in my original answer:
The size of a viewfinder is determined by its magnification. It is measured relative to sensor-size and coverage. The former has a significant impact while the latter a small one, since coverage rarely varies by more than 5%. so you have to normalize it in order to compare it in absolute size.
The absolute size is labelled as Effective Size on the DSLR Viewfinder Size article. This article is fed right from the database and computed live, so it always contains the latest data supplied to me by manufacturers. You can sort by clicking on any column heading. A difference of 0.1 is significant and 0.05 is still easily noticeable. I would not expect to notice anything less than 0.02.
Since you mentioned wearing glasses, there is another number which is rarely supplied by makers and that is the eye-relief point. That is the distance at which your eye can see the entire viewfinder.