This is a follow-up question of this one:
What page numbering conventions are there for front matters (prelims) of books?
If I were to use roman numbers, should they be in small or capital letters? I've seen this changes when using babel in spanish or english with LaTeX, but I'm not sure if this is because of a graphic design guidelines.
This is a matter of stylistic convention, not design rules. Styles and style guides vary from language to language, country to country and publication to publication, but in English-language publishing the consistent practice is to use lowercase Roman numerals for front matter folios (page numbers). The Chicago Manual of Style -- a commonly-used style guide in the United States -- specifies this. I don't recall ever seeing a professionally-published English-language book that used uppercase Roman numerals for front matter folios.
