There is no direct relationship with pulley size and cog count. But the pulley diameter does affect the pulley/cog clearances. As well as the chain length. A common way to fit a larger 1st gear cog with a der not designed to handle it is to use a smaller upper pulley diameter. But exchanging pulleys isn't that simple as their width and whether there's designed in end play (upper SIS pulleys often have this "slop" to help absorb adjustment and manufacturing tolerances) come into play too. Some pulleys have different tooth profiles for the upper and the lower. If in doubt replace with the same model, next best is with the same specs but for the price point. Any replacing that is outside this will be a discovery/trial and error. Andy.