Hellspot, the ATX prefix of the mobo's specify their size, in the same time their upgrade possibility because reasons.
(Small) Micro-ATX: Usually this doesn't have that much, usually 2 to 4 RAM slots and just 1 PCI-e card.
(Medium) ATX: Depends.
(Big) Extended ATX: Usually for 4+ RAM slots and 2+ PCI-e cards. Also 2+ CPUs.
These are, at least, the standard types, there are also middlemen and bigger than EATX.
Micro ATX: Orange
ATX: Green
Extended ATX: Blue