Your dry lot sounds like it should be OK. For steady summer grazing you will need at least 3 well managed acres for 2 horses. This means with adequate rainfall and a long enough growing season, you should not have to feed hay from about the end of May until around mid-October. If you harrow, rotate and manage weeds, three acres will give you sustainable grazing. Count on buying and feeding hay for 7-8 months a year.
With 1 1/2 acres, expect half that amount of grazing time and to buy more hay. Managing a smaller space is trickier. If you have a dry lot and hay to feed, you can graze until the grass gets down to about 4" in length and then put the horses on the dry lot and switch to hay. This periodic change in feed could be hard on their systems though. You will also need to take them off the grazing area when it rains as their hooves can do as much damage to the grasses as overgrazing can. As an alternative, you could restrict grazing turnout to a few hours every day, and offer one meal of hay every evening or morning in the dry lot. Some horses won't touch the hay if the is grass available.
As long as the ditches are clean, and there aren't any municipal rules against it, you could graze them a bit. I would only do it when I can supervise the horses. Do make sure the ditches are not sprayed for weeds and that there is no garbage. You will find the grass doesn't last very long, but it may give you a few hours of grazing once a week or so. Again, follow the 4" rule.