If you are building a new wheel you will need to measure the hub and rim to determine the length of spoke. Here is a selection of spoke calculators you can use to determine spoke length.
http://www.prowheelbuilder.com/spokelengthcalculator
https://spokes-calculator.dtswiss.com/en
https://www.spokeservice.ca/utilities/spoke-length
Once you have determined the spokes size see our selection of spokes cut to length.
If in any doubt please give us a call on 01202 424945 or have a spoke professionally measured.