How to determine sample size for future experiments.

I am measuring the amount of "factor A" for an experiment from two populations (young and old). For each population I have three biolgoical replicates. Th mean and SD for the young group is 4.74 and .49 while the mean and SD for the old group is 6.382 and.3008. I ran a unpaired t-test and teh p-value is .0098. The difference between the actually means are small and I'm wondering if I have a large enough sample size to be confident in this result. When I claculate the effect size I get the Cohen's d value is 3.78 and the effect size r=.883. From my basic understanding, this is a medium effect size, which would support that this difference is of practical significance. Is this correct? Deos this mean I do not have to increase my sample size? From this pilot experiemnt, is there a way to calculate what sample size I need to be confident this result is real?