The strand size is the amount of tension the guy wire can hold. This is not always obvious since the industry often refers to the thickness (like "1/4 inch") but in actuality we would want to record the M's (thousands of pound-force; 6.6M could support 6,600 lbf).
It will be common to have multiple downguys to a single anchor. In that case you will need a unique strand size for each downguy. It's important to go in order from the top to the bottom, based on where the guys are bolted to the pole. Also, if downguys have the same strand size, you will select the measurement again, as many times as needed.
The height stick operator will use a guy gauge tool to measure each downguy in order while the main camera operator will again record the data on the phone. If the main camera operator selects the wrong option, you can scroll down to the "Sizes Of Attached Dn Guys" attribute and simply select the incorrect measurements. This will remove them from the list and you can select the correct measurements from the top of the panel again. Be sure to maintain the correct order still when rerecording downguy strand sizes.