If the glass is out, it'll have to come out anyway, and the retaining brackets are removed you can measure the space and subtract 1/4 inch or less from each dimension and be reasonably sure of the fit. If the glass is still in the frame you have a chance to measure the glass remains before the removal. Even the rubber cushion between the glass and frame will carry an imprint that can be measured if done carefully. Don't get cheap glass, the thicker plate glass is preferable and be sure the glass doesn't have the greenish tint from the UV absorbing iron contamination.