Dell psu for R12 is standard ATX form factor.  after case swap you can also upgrade to any standard non-Dell ATX psu This is by far the most popular PSU form factor and is often called the “standard ATX PSU” Standard dimension for this form factor is:150mm (W) x 86mm (H) x 140mm (D).
I've been thinking of case swapping my Alienware aurora r12 with the RTX 3070, Intel Core i7- 11700F and 550W PSU, but I can't figure out the ATX size of the PSU. Does anyone know this?
Aurora R12, 550w power supply ATX size?