There are online spoke calculators that will tell you what size spoke to order. This is just one of them that is pretty simple if you have the numbers you need.My only concern is the different diameter of the Nexus 7-speed hub. Is there a particular length of spoke you can choose or do you have to cut them individually? That's the part that bothers me.
https://bikeschool.com/resources/spoke-calculator
This one has a database of hubs and rims to get you the numbers, but I usually measure the hub myself and look to the manufacturer to get the "ERD" (Effective Rim Diameter). I've measured that myself before too and it worked. The pages with the calculations will give descriptions of what these numbers are.
https://kstoerz.com/freespoke/
You can order spokes in any length you want though be aware that sometimes changing the requested length by 1mm can change the price significantly. I've ordered them a touch long before to get a price break. I'm guessing it has to do with "standard" lengths vs custom lengths.