Let's put things in perspective:

A football field is 300 ft long, not including end zones. If you can find a football field, place the plane on the ground at one end, then look at it from the other end to get a "visual picture" of it's size at 300 ft away.

OR

At your field, take a piece of rope that's a specific length, like 10 yards (30 ft) , and count out the 10 yard (30 ft) lengths until you reach 500 ft. Again, place your plane on the ground at one end and get a "visual picture" of it's size from the other end.

Either of these methods should give an approximate altitude, based on the plane's "visual size", while it's in the air.