I don't know where you are, but in the UK there are generally two sizes of outside hose tap union in common use.
One is 0.5 inch BSP male and the other is 0.75 inch BSP male. If you have a 0.5 inch BSP male hose union tap and your hose has a 0.75 inch BSP female connection, just get a reducing bush (0.5 inch BSP female by 0.75 inc BSP male).
Note: All these BSP threads referred to are parallel, not tapered, so you generally need fittings with sealing washers.
Brian