My question is: will 1/4 inch sash chain fit in sash pulleys that are 6/16 inches wide? Or is there some other measurement of the sash pulley that I need to take?

The windows in the rear of my circa 1900 house do not have a pulley and weight system. I call them guillotine windows. I have purchased pulleys to install and now need to buy chain. Rope is used on the windows in the front of the house, but I want to use chain in the back, since I won't be installing the cut-out at the bottom to reach in and replace the rope.

Each sash probably weighs less than 15 pounds. (I haven't weighed them yet but the front sashes weigh that much and the back windows are smaller.) The sash wheels measure side-to-side 6/16 inch.

Looking at websites that sell chain for this purpose, the options seem to be link widths of 11/32 (#3) and 1/4 inch (#8). And the width of the chain seems to have more to do with what the chain is made of, the finer chain being made of brass (more expensive) and the others made of steel.