There are two (maybe three) major things to look for in a viewfinder. The first and most important is coverage. Viewfinders provide an approximation of what the sensor sees. While they are looking through the lens, they don't necessarily match exactly with the sensor size. This is why there is normally a % coverage associated with the viewfinder. 100% coverage is the best, but often on cheaper cameras, it is more in the mid-90s which means you don't see some of what the sensor will see.

The second is pentaprism vs pentamirror. This question has a lot more useful information about the specifics of the difference, but the short version is that a pentamirror is cheaper and lighter, but passes through less light and is less clear. A pentaprism is higher quality and brighter, but more expensive and heavier. Pentaprisms are generally better for there superior visibility.

The third, though only important for people who wear glasses, is the amount of diopter adjustment. Diopter adjustment allows for some correction for vision issues to allow more direct use of the viewfinder.

As far as your original question about the "bigness", that really isn't measurable. How big it seems to you is going to depend on how you look through it to some extent. The biggest recommendation I can think of is to stop somewhere like a Best Buy or some camera store and take a look at the demo models to compare for yourself. The way you hold the camera and how comfortable the viewfinder is for you to use is going to make a far larger impact on ease of use than "bigness".