The 40 will be somewhat smaller capacity than the 45. If you look for "specs" you will often see the capacity of a pack listed in cubic inches or in liters, or both.

Whether either one of them will hold what you consider to be a week of clothing is impossible to say, because what each person would call "a week of clothing" can differ enormously.

The best way to discover if your clothes will fit is to take all your clothes that you want to carry and put them inside it. You can also put the clothes into a box and measure how many cubic inches they fill. This will get you a fairly decent idea of whether the pack will hold them.