1st find a way to locate your center channel just below the TV/Monitor. Fairly easy if monitor is wall mounted. Mine: I built a 6" high riser to sit on my equipment cabinet below. TV on the riser, center channel below/within the riser.
2nd, match your center speaker’s sensitivity close to your FL/FR speaker’s sensitivity. You can adjust front/center volume in your AVR but avoid extremes.
3. Decently accurate frequency response, don’t ask it to produce too much bass, that helps keep it smaller.
4. Directivity and Port or NO port, I go for no port.
5a. my 1st, very nice was a small Bose VCS 10, surprisingly good, see vents at front top, position needs to allow this.
5b. now: Klipsch R-34C, very nice, no ports, very high sensitivity, not a great choice to pair with low sensitivity Front mains. Wife loves the copper drivers when grille is off.