I have never had a problem with dandruff until I started using Rogaine.

When using Rogaine I get dandruff the size of dimes...I am not kidding.

Any suggestions?

this is a common problem with rogaine/minox because rogaine drys the scalp with many users which in turn leads to danduff. Try using a shampoo like Nizoral (which I hear is very good for hair loss and leaves the scalp feeling great) or an anti-dandruff shampoo with Zinc a few times a week and see if that helps.