I am gonna take a guess at your motor being a small 1ph item with a belt drive to the compresser. I cant give any definitive figure for capacitance values but if you are lucky it might be stated on the motor nameplate, you are looking for a figure with what looks like the symbol `uf` after it but with a longer tail on the u. This quotes the value in microfarads. Alternatively the easiet way is track down the maker or sales agent, give them a phone and ask them. If its a newish compresser you may even be able to get the info from the compresser makers and buy it from them as a spare part.