Take into consideration a few factors. First, you need to see what size your bedroom is and whether it can even fit a Queen sized mattress or above? If you have narrow elevators or staircases, you'll need to consider box springs and mattresses for kings to be two Twin XL instead because a King sized mattress may not fit.
Then you want to see how big you are and if you're sharing it. If you're six feet or taller, consider getting a longer mattress, such as a Queen or King-sized mattress. The Full XL may work, but it also depends if you're muscular or heavyset and will need additional width than it can provide.
If you're going to be sharing this with a partner (or pets), then go Queen-sized or above when possible, unless both of you (and possibly your pet) are of smaller stature. Those extra few inches of width go a long way to not disturbing your partner.
The final piece is your budget. There's a significant price difference between each size upgrade, and if you're looking between a Queen or King-sized bed, you'll quickly notice that it could be a few hundred dollars more for that extra size.
Accessories and bed linens are usually priced based on the material amount, meaning that having a King-sized mattress will cost more in the long run due to its larger size. It all comes down to comfort.