A couple tricks I have used to figure out my pool in gallons.
1.) If you ever decide to drain your pool, before refilling put the hose(s) that you use to fill your pool into a 5-gallon buck and determine how long it takes to fill the bucket. Then when you fill the pool, keep track of the time when you start and finish the fill. You can then calculate how many gallons your pool is based upon how long it took you to fill it up.
2.) An even easier method is to determine size based upon how much your FC raised after adding chemicals. Using your test kit, test your water and log it all into the pool calculator. Do a best guesstimate as to the size of your pool. After testing, add the exact FC it tells you to add in order to get to your target FC. Circulate water for about an hour, then test your FC levels again and determine how much your FC level actually raised. Go back to pool calculator and under "Effect of Adding Chemicals" plug in the amount of chlorine you had added. Now play around to the pool size in the calculator until the "effects of adding chemicals" matches your actual change in FC level. I would do this at night, since you don't lose chlorine during the hour you circulate the pool. Also, try to use a fresh batch of chlorine.