To find the right nozzle size, you will need to know your GPM and desired PSI. Choose the desired PSI on the top column, then choose as close to your GPM as you can get without going over.
Example: Desired PSI 3000 x 1 Nozzle at 5.5 GPM = 6.0 orifice nozzle
The easiest nozzle chart I've found is through Giant Pumps. Input your pressure and flow rate to easily calculate your nozzle size and reaction force. Giant Pump Nozzle Chart
For a more detailed explanation on how to pick the right nozzle for your pressure washer using the nozzle chart, watch Marco's video.