Sample sizes for testing differences in an unpaired ttest
A popular effect size for ttests (unpaired or paired) is Cohen's d which represents the standardized difference between two group means. The highlighted rows in the table below represent small, medium and large effect sizes as suggested by Cohen (1988). This table gives sample sizes required in each of the two groups.

90% 
80% 

d 
1tailed 
2tailed 
1tailed 
2tailed 

0.1 
1708 
2096 
1234 
1568 

0.2 
428 
525 
310 
393 

0.3 
191 
234 
138 
175 

0.4 
108 
133 
78 
100 

0.5 
70 
85 
51 
64 

0.6 
49 
60 
36 
45 

0.7 
36 
45 
27 
34 

0.8 
28 
34 
21 
26 

0.9 
23 
27 
17 
21 

1.0 
19 
23 
14 
17 

1.1 
16 
19 
12 
15 

1.2 
13 
16 
10 
13 

1.3 
12 
14 
9 
11 