Trying to measure a solid by volume is just about impossible to do accurately. It depends too much on the grain/crystal size, and that's going to change by the brand of epsom salt you buy and such. If you don't have an accurate mass balance, you could try 1 1/8th teaspoon and see what happens. Is this a value you need to get a particularly accurate measurement of?
You could also construct a mass balance of your own and use 2 clean new pennies on one side (5g) and add epsom salt to the other side until it balances. That's probably not worth it though unless you've got a small kid to science-ify or you really are determined to get 5g without much error.