The most conventionally sized motherboard is a Standard ATX which will be most comfortable inside a mid-tower and a full tower. However, if you want a small form factor machine, the Micro-ATX and even Mini-ITX depend on the case size. These smaller boards will have space for your CPU, RAM, NVMe SSD and other essential components, but may cut out other connectors to save on the silicon. On the reverse, you can opt for an E-ATX (or extended ATX) larger than a standard model, allowing for more room to build provided your chassis is large enough.