If a common version of a particular card was ever released on Magic Online, any version of that card is legal in this format.
Example: Counterspell was a common card in Seventh Edition Core Set online. Counterspell was reprinted in Masters Edition II with an uncommon expansion symbol. Both versions of the card can now be used in pauper.
Other than that, the usual rules for constructed decks apply:
- Minimum of 60 cards in the main deck
(There is no maximum size for main decks)
- Up to 15 cards in your sideboard, if used
- No more than 4 of any individual card in the main deck and sideboard combined (with the exception of basic lands)