Well the first rule of thumb with food size for crickets is that they should be no larger than the
distance in between their eyes.

As far as how many per day per frog.
There is a good debate over that issue.
I fed mine that were similar size 3 to 4 crickets every other day. And dusting with a vitamin powder with out
D3 one day and the next day dusting with a D3 alternating since some vitamins block absorption of D3
And I also gut load my crickets with a calcium rich cricket food from flukers and fresh carrots.

Now I would place some crickets in a small glass candy dish to keep them from running free
and biting my babies.

Then once a week I would also tong feed to make sure everyone gets their fair share.
I like this method the best since I love actually feeding my babies.
Another way is to feed in a separate container. But you have to handle the frogs for this and
this can stress them.

Good luck and enjoy you babies,