Determining sample size in a factorial repeated measures ANOVA using GPower

Hi all

Can anybody tell me which F-test option in G power is appropriate to find out the sample size for a 2x2x2 repeated measures ANOVA?

The three within subjects factors are A, B & C.

I am interested in finding out how many subjects I will require to find a main effect of A and an interaction between A&B.

Any help would be greatly appreciated. (I can only find advice on mixed factorial ANOVA and so I am not sure how this applies).

Many thanks in advance

(PhD student losing the will to live).