What size filter is for the tempstar high efficiency 5000 furnace?

Can't you find the old one? Where does the old one fit? measure the height and width.

There was no old filter and the bracket is pretty beat up. I'm pretty sure it's a 16x25x1. But it just doesn't fit right.

That's a fairly common size. Some filter frames have a blade spring that must be depressed. It allows the filter to snap in. What about it that doesn't "fit right"