Determine the area you need for your DSUB connector.
IF you're thinking of making more than a few units, maybe pick 2-3 models of DSUB connector from different manufacturers, or 2-3 different footprint formats (in case one model is no longer stocked, you'd be able to fall back to the other model)
Let's say each connector is 5cm x 3cm...
Determine if you can design your circuit board as "pass-through" ex: dsub -> [ box ] -> dsub
or if you prefer to have both connectors on same side.
If that's the case, you'd need 2 x dsub width + some room between them + some room on the sides... so for example 2cm room on left+ dsub + 1cm between + dsub + 2cm = 15cm
Now go at various distributors of components and look at project boxes that meet your needs.
See Digikey, Mouser, Farnell/Newark , RSComponents etc
You have your constraint (width and height required for the dsub connectors), pick the height to be higher than your dsub + whatever high components you have on board
Some cases don't have screws, they have channels ( [ -- ] )in which you slide the circuit board. This is common for extruded aluminum boxes. If you pick such a box, you'll need to leave a few mm on the sides that will sit in the channel.
A lot of cases will have two standoffs , some will have 4. Typically, they'll be on diagonally opposite corners of the case.
You'll have to be careful of the dsub position if it's the kind soldered on the pcb, so the body of the dsub conn. won't be above the screw hole making it hard to screw the pcb to the case.