Ok first of all, you have to gauge how many carbohydrates you wish to consume. Lets assume you wish to consume 100g per day - its a good number to start with. So if you want to consume all your carbohydrates from sweet potatoes, you would consume roughly 500g of sweet potatoes because that would roughly equate to a 100g of carbohydrates. I'm not going to pretend I know how many grams your sweet potatoes weigh. The best thing to do is to get a scales weigh them, then look them up.
This will guide you for how many carbohydrates are in fruit and vegetables. Its especially useful when eating starchier vegetables or fruits which are high in sugar.
Take a note first time. You'll then be able to intuit how much sweet potato, banana, potato or whatever else it is that you want to consume.
Also note: you may want to consider energy levels, sweet potatoes are a good post work out meal.