just use the same size cup the pots are in and fill it a third of the way up i usually hold it up and poor lil by little when there young until its drips from the bottom, have u got any perlite in your pots it helps with drainage and prevents root rot, if not buy some before you re pot them also is there holes in the bottom of your cups?
I have perlite chilling, however, I honestly think I probably don't need it for the soil I have. I do have holes I made in the cups I am currently using for my sprouts, but I'm not watering until water starts seeping out yet. I didn't think that was a smart thing to do for newborns since I would think they would drown if I used that method. Fortunately, thats probably the reason why the soil keeps getting bone dry and my insufficient watering methods are causing the sprouts to become the way they are. I'll wait 2-3 days depending on their progress with no water, afterwards, I'll go ahead and water each cup until water barely starts running out the ends. Again, good tips I hope this is the reason why my little crop is the way it is.