When it comes to deciding on the size of your PC case, there are a few things you need to take into account. The first is the form factor of your motherboard. This will dictate the maximum size of your case. If you have a full-size ATX motherboard, then you'll need a full-size tower case. The next thing to consider is how many hard drives and/or optical drives you want to install. Each drive takes up some space inside the case, so if you plan on installing multiple drives, you'll need a case with enough room to accommodate them. Finally, think about how much expansion cards (e.g., graphics cards, sound cards) or other components you might want to add in the future. These also require some space inside the case, so it's important to leave some room for them when choosing a case size.
In general, there are three main sizes of PC cases: mini towers, mid towers, and full towers. Mini towers are the smallest and most compact option; they're typically best suited for basic PCs or those that will be placed in tight spaces (e.g., under a desk). Mid towers strike a balance between size and expandability; they're big enough to fit most motherboards and multiple drives/cards, but not so large that they become unwieldy or difficult to move around. Full towers are the largest option; they offer plenty of space for all kinds of expansion and customizations, but can be quite heavy and bulky as a result.