4' x 4' x 8" Router Build

[JAZZCNC]

The thicker the better as it significantly dampens vibrations which translates into cleaner cuts and longer tool life.
Don't be messing around with sand etc because while it dampens vibrations slightly the cost in performance due to the extra weight is too expensive for the returns.
Spending a little extra for Thicker steel will pay dividends in quality and stiffness.

For a machine this size I'd go with 60x60x5 as a minimum. It will give you a much stiffer machine than using 50x50. I've built machines this size using both sizes and 60x60 is the better choice.