How much evaporation can one expect from a swimming pool that is 25m long by 5m wide and with an average depth of 1.5m in Dubai? How much less evaporation if the pool is 3m wide? What is the implication in $ on the monthly utility bill?

Some variables are missing in your question, like the humidity ratio in the air and the velocity of air above the water surface. Here we find the needed formulae to calculate the expected amount of evaporated water.

The amount of loss depends on the pool surface area. With an average evaporation rate of 167mm per month the larger pool water loss is 20.8 cu.m per month and the smaller pool is 12.5 cu.m per month, giving a water saving of around 8 cu.m per month.