Perhaps the best analogue one can think of might be a hang glider. These carry a fair amount of weight in themselves that has to be factored in to the supporting lift, but then on the other hand the human skeleton isn't designed to support its own weight in this manner and arms (& legs) may well need more reinforcement to support the weight in any event.
So; the basics. The 'average' wingspan of a hang glider is around 10m, a little less for more advanced units. The average human arm span is roughly proportional to height, so say around 1.8m for the average person. That's nowhere near enough, but then it's important to note that arms aren't all that bats use to spread their 'wings'.
A bat's arm pretty much only extends to the front middle edge of their wing. The rest of it is supported by elongated fingers. In practice, that means that if you have fingers grow out to around 2.5m, and you have the arms roughly double in size, you might be able to get enough surface area for gliding flight for a human, although there are a lot of other factors to consider.
Another way of putting that last statement is that a human would need to change shape, develop muscles in different areas (to control the shape of the wing), and reinforce lift-bearing sections of the skeleton for any of this to work.
Put yet another way; depending on the final shape, you may be able to get away with less, or you may need more. These figures also assume that the length of a human, particularly between outstretched hand and foot, makes the surface area of the wing correlate to that of a hang glider. This may well not be the case.
Above all, remember the cube square law; the volume (read as mass) of a creature increases much faster than size, meaning that you can't just take the proportion of wing area of a bat's body and scale it up to humans 1:1 as that won't give you anywhere enough lift. You'll probably find that your gliding human will be much more lift area than body for that reason.