A bigger rear sprocket will give you more speed, and a bigger rear sprocket will give you more low end torque.
but, when you gain top end speed with a smaller sprocket, you lose low end torque.
when you gain low end torque with a bigger sprocket, you lose top end speed.
wheel and tire size can also change your low end or top speed.
since you're running 28" wheels, I assume that the tire is about 29" tall?
you can't get much taller than that for a bike tire.
if you put a 26" rim in it, you;d lose a tiny bit of top end speed with your 36 tooth gear.
I think if you wanted to go smaller than a 36 tooth, you'd have to have a custom made sprocket, or modify a small front pedal sprocket to work,
I have a bmx bike with a 28 tooth front sprocket, so they are out there...getting one to work is another story.