Proper size of tomato for replanting would be when after the cotyledons tomato grows first leaves. That is the moment when main root starts developing tiny side roots and moving from small container to bigger won’t be so "painful" for the plant.
Make sure to water tomatoes evening before replanting. Morning right before replanting water soil in new containers. Make a hole in the middle of soil in each container and replant tomatoes.It is very important not to press, squeeze or damage tomato root during replanting.
If everything is done properly, tomatoes would quickly recover. When replanting tomato, keep leaves at least 1 cm above soil. Then gently press the soil around and water well. To help tomato recover quickly, keep it a bit shaded first day with agro textile.