I know mine are 10", but I think the brake size is a function of the axle used. My axle is rated at 3500#. Perhaps, fir your axle is a Dexter, you could find the brake size from their website.
Otherwise, simply open up one of the drums and measure what you have. It's not a complicated thing to do, especially if you're planning to replace the brakes anyway. I think you can get them shipped to you in a fairly short time. I got mine through e-trailer as I recall.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew