I have a day & Night model 565AB060 air conditioning unit. I need to the size (tonnage) of the unit.

Should be 5 ton that is 60,000 BTU

How the cooling tower tonnage is related to chillers tonnage?

cooling tower is nothing but a air washer,,

rule of thumb ==tr of chiller+0.25xtr

by equation.
parameters are.

entering water temp.-
leaving water temp
wet bulb temp of ambient air.

approach=5-10 °F
Evaporation losses are about 1% by mass of the circulation rate for every 10 °F (5.5 °C) of cooling. Drift losses are around 0.25% of the circulation rate. A blowdown of about 3% of the circulation rate is needed to prevent salt and chemical treatment buildup

. Water circulation rates are generally 2-4 GPM/sq. ft (81-162 L/min m2) and air velocities are usually 5-7 ft/s (1.5-2.0 m/s)

heat rejection rate=15000btu/hr per tonn