For simple shapes try one of these:
If your pond is U shaped, this formula should give you ball park estimate of the area. Length ft x width ft x 0.785 = square ft. square ft divided by 43560 = acres.
If you have a round ended long triangle, try this formula: length ft x width ft x 0.523 = square feet. Subtract a little square footage for the rounded corners at the widest base depending on how you mesure it.
If you have an oval pond try this formula: lengthxwidth x 0.785.
If you have a triangle shaped pond: length x width divided by 2.
As a double check, it is best to compare your results with the other suggestions.
To figure the largest it could possibly be consider it a rectangle or square and multiply the longfest length x longest width. All shortages and deviations from this basic rectangle need to be realistically estimated and subtracted from the maximum possible size.
