You should be able to find a replacement speaker easy. No issues.
The "5.25" speaker size refers to the cone part (Actual speaker part that moves up and down, not the outer frame). Im looking into replacing my 5.25 speakers in my Axis but Im thinking of increasing the diameter to accomadate a 6.50 inch speaker for more sound. Like the say, the bigger the better. LOL.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor