I'm new to Magic and am not familiar with how standard, modern and legacy tends to work. I'm sure I'll learn shortly, but when I'm adding and changing cards in a deck I'm building on this site, it would be great to see which cards are causing the format to flip from one format to another.

Is there a way to put a S, M, L or other symbols beside the individual cards? I realize cards change with time and some get banned or other various things. Is that something that can be added to the site somehow, perhaps for nubies like me? Maybe even just the legacy ones or the ones that are easier to pin-point from a database perspective?

May I suggest that you do a search in the card database that will only include cards from the format you want to build in. You can add cards to your deck from those search results as well.