How deep is this case? I am seeing reports of 17.7, and 19.88.

The product information does not specify the depth of the case.

will this support smaller motherboard sizes or only ATX?

This chassis supports 12" x 9.6" ATX, 11.2" x 8.2" Mini-ATX and below motherboards.

This chassis has 12 internal 3.5" drive bays.

The maximum CPU cooler height supported by the Rosewill 4U Rackmount Server Chassis is 158 mm.

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Unleash the Potential of a Full 4U Platform: Experience superior expandability and thermal performance. This 4U chassis is engineered to support massive full-size ATX motherboards and high-wattage CPU coolers, providing a robust foundation for high-performance computing and storage systems.
Massive 12-Bay Storage Capacity for Demanding Workloads: Configure up to twelve 3.5"" HDDs or SSDs, delivering petabyte-scale storage ideal for media servers, data archives, and enterprise backup solutions. This chassis transforms into the central hub for all your critical data.
Efficient High-Airflow Thermal Management: Two 120mm PWM fans generate a robust and focused airflow path. The generous 4U internal space prevents heat buildup, ensuring your high-end components run cool, quiet, and stable even under sustained heavy loads.
Front-Panel USB 3.2 Gen Type-C for Ultimate Convenience: Achieve transfer speeds of up to 10 Gbps with the front-accessible Type-C port. This modern connectivity solution drastically reduces backup times and streamlines workflows with compatible high-speed external drives."