i have a 2006 f150 2wd regular cab. bought it last may, then in august i deployed. im tryin to buy speakers for it because i blew the stock speakers on an aftermarket HU. i dont kno what sizes they are. best buy said that 6.5" and 5.25" wouldnt fit and i think best buy is a bunch of tards.
you should be able to buy a plate that will fit in the stock hole but allow a different size speaker. I replaced my stock speakers with ones from crutchfield.com and they sent me plates for free that fit my new speakers.
The stock speaker size is a 5x7/6x8. You could either replace with those or go round and get 5.25 or 6.5's. You have to ask yourself what you are going for and how much you want to spend. If you just want to replace them with a simple upgrade, buy the stock replacement. If you are going to build a full aftermarket system some will agree to go with round speakers.
On crutchfield (even though the prices are high, it is a great place to shop/compare) if you punch in your vehicle it will display all speakers that will fit. This will include 5.25's and one pair of 6.5's(alpine type r) because the include adapter plates for free. If you dont buy from them you can purchase adapter plates online or just make some out of mdf.
Do you plan on using the stock headunit or aftermarket? Do you plan on amping them?