just use the formula : R= U/I
U is your voltage you want to get rid of. probably 0.5 or something since you will need to go from 3.7 to 3.2 V. and the I is probably 0,02 A if you place all your led's paralel.
wich brings us to : 0.5/0,02 = 25 ohms and about the wattage it will come out at 0,01 or something so just keep it as low as possible to save space and money.