The Full Towers are 22” – 27” long. They have 5+ expansion bays for adding extra storage, more drives, etc. You will be able to fit EATX, ATX and smaller motherboards with ease. They can accommodate up to 10 or more HHDs and SSDs and can easily accommodate multiple graphics cards without any problems.
The Mid Towers are 17” – 21” long. They are the most common size case used by people. They have 3 – 4 expansion bays for external storage drives, graphic cards, etc. You will be able to fit ATX and other smaller form factor motherboards like Micro ATX and Mini ITX. You will be able to accommodate up to 6 or more HHDs and SSDs and two graphics cards comfortably.
The Mini Towers are 14” – 16” long. They have 1 – 2 expansion bays for external storage drives. You will be able to fit micro ATX and Mini ITX motherboards in this case. You will be able to use up to 4 HDDs and SSDs and only a single Graphics card comfortably.
The Small Form Factor (SFF) cases are the latest entry slowly growing in popularity. They are less than 12” long and come in various shapes. They have only a single expansion bay. It can use a single HDD or SSD and only support the Mini ITX motherboards. They do not support the standard-sized graphics cards and require a small form factor graphics cards.
Always check whether the case you’re buying has enough room to accommodate your motherboard, graphics card, and CPU cooler. You need to check the form factor in case of motherboards, length and height in case of graphics cards, height in case of CPU air coolers and radiator size in case of liquid coolers. You’ll easily be able to find out about the dimensions of these products in either amazon or their respective manufacturer’s website.
Here is a full in-depth article on various motherboard sizes.
HDDs, SSDs, and RAMs will be able to fit into any case. They also have a defined size so their accommodation won’t be a problem.