Alright, assuming that the sliders you have are 1/2" and 1/4", you want to have it 1/8" shorter than the space between the two wood pieces. Assuming that they are exactly right, the space between the two wood pieces is 19 inches, so you want the plexiglass to be a LITTLE smaller than the space, but enough so that you can easily snap the glass in. So if you think you can snap in a 19 incher, that can work fine, but that will be a very tight fit, so it might be better to get it cut to 18 7/8".
Bear in mind I am not the best at construction, but this should work just fine. Getting a second opinion wouldn't hurt, but keep in mind that there still needs to be at least some overlap on both sliders so it fits in. Mine unfortunately got cut a little longer down than I wanted, but it still works.