A properly fitting boot should feel like a firm handshake.
While sitting, place your foot in the boot. Stand up and step down into the boot. You should feel some resistance in the heel area as you step down into the footbed. This is the heel counter. If your foot slides right past this area, it is likely too large.
Once your foot is in the boot, your heel should have 1/4-1/2" of slip and the vamp should wrap the top of your foot like a firm handshake.
If the boot is a bit tight, but not painful, it should break in over a day or two as the leather molds to your foots shape.