When making your bed, it's a good idea to leave some room around your mattress so that you can easily tuck in your sheets, blankets, and other bedding without them becoming too tight or difficult to manage.
As a general rule of thumb, you should leave about 3-4 inches (Way too much. 2" at the foot and head of the bed is fine and 1"- 2" on each side) of space around the sides and foot of your mattress. This will give you enough room to tuck in your bedding without it pulling too tightly or causing wrinkles. However, it's important to keep in mind that this can vary depending on the thickness of your mattress and the size of your bedding.
If you're using particularly thick or heavy bedding, you may need to leave a bit more room to accommodate it. Similarly, if you have a particularly thin or lightweight mattress, you may be able to get away with leaving a bit less space. Ultimately, the key is to experiment and find the amount of room that works best for you and your bedding. (Comical as it assumes you can adjust your bed size?)