JorgeP, Here's the specs we use when using the IMB. We use the USPS4CB font from the USPS RIBBS website. Perhaps this will help you.
FP: IMB Above Address Block
16pts ; Space Below = 7 pts
FP: IMB Below Address Block
14pts ; Space Above = 3 pts ; Leading = 10 pts ; Tracking = 2% ; Kerning = YES