Start low and if you want more you can add more to the fermenter.
I don't know. adding fresh ginger to the fermenter might well result in biological activity that you did not intend. Lots of critters living on/in ginger.
I buy on, but in? I added a pound of peeled ginger to a fermenter once and didn't see new biological activity. Took a year for the ginger to mellow out though.
For reference I used a pound and a half of ginger for 310 gallons of beer and ginger is noticeable at that level.
Seems like that works out to about 1/4 oz for a 5 gallon batch. Thanks for the point of reference.
On Tap/Bottled: Golsch, Iliff IPA, Hopfenbier, Salted berry cider
Fermenting: Rye Cream Ale
Up Next: G Pils, Maibock