I'd probably recommend using
at least a 6 at least for this scale Rodrigo - I use a 3 for some painting down to 28-32mm miniatures so that's definitely too small for the same areas on a 90
If you're used to only using rounds I'd highly recommend some flat brushes* as well. This is an area where I don't think you need to go to the expense of buying Kolinsky or sable, synthetics are usually more than good enough. And in case you didn't know, there's no standardisation with brush sizes so how big they are depends a bit on the brush style - a 1 from two makers can be very different, both in length and width.
*A filbert or cat's tongue (langue du chat) shape generally works better IMO than a bright or a flat.
Einion