Any size jar will work. The quart size jar fits nicely in an empty medium while the gallon fits inside an empty deep. Depending on the weather, and how fast they take the syrup, depends on what size you want to use.
If they are really taking the syrup, like a package building comb, then the gallon is best because you won't be filling it every/everyother day, and they syrup will be consumed before it goes moldy.
On a established hive with stores left from winter, the consumption will be slower, and the quart size works better and will be consued before it goes moldy.
There are many other things better to spend your money on than a feeder, when glass jars are free :lol: