Since you are asking about this outdoor bench, the key number to check is the usable seat depth, not the overall bench size. For most outdoor benches with a backrest, a seat depth of about 17 to 20 inches is the sweet spot for an average adult. That measurement is taken from the front edge of the seat straight back to where the backrest meets the seat or cushion.
That 17 to 20 inch range is generally deep enough to let you sit all the way back, relax your shoulders against the backrest, and still have a little space behind your knees before the front edge. If the seat is only 15 to 16 inches deep, it can feel more like a perch. You may end up sitting upright, sliding forward, or feeling like you cannot fully settle into the backrest. If the seat is much deeper than about 21 or 22 inches, taller people often enjoy the extra room, but shorter adults may feel the front edge pressing into the back of the knees or find themselves slouching just to use the backrest comfortably.
So for an average adult, yes, an outdoor bench seat in the 17 to 20 inch range should be deep enough to lean back comfortably. If you are around 5 foot 6 to 5 foot 10, that range works well. If you are shorter, look for something closer to 16 to 18 inches. If you are taller, 19 to 22 inches tends to feel more supportive.
One more detail worth checking is whether the listed depth includes cushions. Thick cushions can reduce the usable seat depth by an inch or two, so a bench that measures 19 inches without cushions may feel closer to 17 inches once padded. Also, a slightly curved or angled backrest can make a shallower seat feel more comfortable than the raw number suggests.
If the product listing does not show seat depth separately, measure from the front edge straight back to the backrest. That is the number that tells you whether you can lean back comfortably, not just how wide the frame is.