First of all, if I understand your situation correctly, going with 2 of the smallest mice from the new batch is closer to a 15 gram jump in meal size than 20g.

However, that isn't as important as what is the proper size meal for your snake. Using a single one of the largest mice will result in a meal that is somewhat below 10% of the snake's weight. Using 2 of the smallest ones will result in a meal that is just above 10% of the snake's weight. Since 10-15% is often recommended (and it has been working well for my snake!), I'd go with 2 smaller ones. If you always take the 2 smallest remaining mice, the meal size will grow with the snake (probably not at exactly the right rate, but it is a start at least).

Plus it has the added benefit of keeping the same feeding routine of 2 food items.