Which motherboard size is smaller?

The ITX motherboard size is indeed the smallest among the options provided. ITX stands for "Information Technology eXtended" and was designed primarily for compact and energy-efficient systems. This form factor typically measures 6.7 inches by 6.7 inches, making it ideal for small form factor PCs, where space is limited, such as in home theater PCs and compact desktops.

In contrast, ATX, BTX, and XL-ATX motherboards are larger. ATX boards measure 12 inches by 9.6 inches, BTX boards vary in size but are generally similar or larger than ATX, and XL-ATX is even larger, at around 12 inches by 13 inches or more, designed to accommodate more features and expansions. Therefore, when comparing these options based on size, ITX is the smallest and is frequently chosen for builds that prioritize a compact design.