We recommend a 6’x6’ for the 4x4 machine and 6’x3’ for the 4x2 machine and should be rated to support 350+ pounds of weight.You can reference the X-Carve Pro overall dimensions to ensure your table is adequate.
The most important part is the feet have a stable surface and there is room to reach the electronics at the back left of the machine.
It is helpful to have enough space in the front for the HMI, tools for setup and enough clearance in the back to load and unload 4x8 sheets.
While a steel frame can be sturdier, any vibration or wobbling from the table can translate back up to the carve. Some people have also built torsion boxes to help with vibrations: https://www.finewoodworking.com/2007/05/15/build-a-torsion-box-assembly-table https://www.core77.com/posts/26334/A-Look-at-Torsion-Boxes