the wheels can be as big as you want honestly, its the tires you have to worry about, and just a guess, but you might be able to go up to 31s or 33s on a stock truck, depending on the wheels you get, you might have some rubbing issues when turning the 33s
I'd say if you want to upgrade wheels as safely as possible, look at what other wheel sizes your car comes with from the factory. For example, what size are the factory premium wheels if you have base model wheels? That would be the safest size to go with as your car was made to be equipped with that size.
For your 05 F-150 I wouldn't go bigger than 18" wheels. I'm guessing with your XLT you have 16" wheels currently. You can go bigger, but you will sacrifice ride quality. The tires you choose are important as well.
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