The only critical dimension is that it's a standard mtn 25x1.xx rim (559) rim. That's critical because affects brake height and any change can take the rim beyond the reach of your brake shoes. Rim width is the second factor, and here there's more latitude & you might want to choose the rim width according to the tire width you plan to use.
Here's a link to a chart that will give rough guidelines.
Note that rim width can also affect the brakes, since braking is best if the side of the rim is the right distance from the canti post. However there are ways to compensate, so fee free to change the rim width to best suit your needs, and you'll compensate the brake later.