1- It doesn't matter or affect fruit size.

2- First, good luck with the outcome of your cross. Second, it won't affect the fruits of what you're currently growing. Your Gems will be normal Gems and your Spaghetti will be normal Spaghetti.

The seed from the cross pollination, but not the fruit, can be different (size, flavor etc.) because its genetic material is from the two different parents. This is not a problem with most vegetables because the fruit or the plant is eaten, but it is a big problem for corn, where the seed is the edible portion. You can ruin your sweet corn by planting it next to field corn, a problem my neighbors across the street have because their back yard abutts a huge corn field.