Which radio library is that you’re using?
If you can’t find someone else having already made a smaller library, you’ll have to do it yourself. I’ve got a couple of cut down libraries in my github where I did just that (TinierGPS and a cut-down version of the neopixel library). A lot of libraries, particularly ones from vendors, tend to focus on exposing maximum functionality and making it as “easy to use” as possible, at the expense of size.
Do you have enough pins to get serial and interface with the RFM22B on an attiny 85? You only have 5 pins, and serial requires 2. And to get serial to work on the '85 you may have to tune the internal oscillator - it’s only factory calibrated to +/- 10%, and serial needs like +/- 2% or so. So it may or may not be close enough to work without calibration (and you certainly don’t have enough pins for a crystal) (and it’s a software serial implementation, so you can’t send and receive at the same time).
I’d consider using an attiny841 instead of an 85 - the 841 has two hardware serial ports, and enough pins that you can use a crystal for accurate timing and have plenty of pins for the radio and serial. Or an attiny1634, if you can’t get the library size down, and still want to use a smaller chip.