| If they're in a 10 they should be fine for a bit, depending on how many fry you have. I usually spawn in smaller tanks, so if I have a large spawn have to move them fairly early. As long as it's done gently enough they can easily be moved after a couple of weeks. However, there's really no need till they start looking crowded, or the tank starts getting dirty really quickly, or water quality's going downhill faster than you can keep up with, or they aren't growing as fast, all of which also depend on your maintenance schedule, etc. My second spawn ever, I raised 125 bettas in a 10-gallon tank (my first spawn was 15 total, so I was caught a bit off guard!) I was doing water changes 2x a day, jarred the boys soon as they were spotted, so they were all perfect, no stunting, no issues, but not something I would ever recommend (I actually had no idea there were quite that many till the boys were all jarred and I got around to netting out and counting all the girls... just that tank was getting dirty REALLY fast lol... they looked less crowded than you would expect!)
Umm... so in a less confusing summary, you can move them early if necessary, but there's no point in moving them till they're big enough for the number of fish in the tank to need the extra space. Or if you'd like to be able to change the water less often. When they're really small they do better without too much space since they're more likely to run into their food and don't waste a lot of energy swimming to find it, but once they're decent at tracking it down it doesn't matter that much as long as the water is clean and fresh... obviously easier in a larger tank. | 
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