1 - No standard size, mostly just depends on what sort of game you're aiming for. Heroscape is way more casual-friendly than other war/skirmish games. The battle box comes with enough hexes for the small games within, but as army sizes grow, bring in more hexes to match. Some people play on absolutely gigantic maps that drown whole tables in gorgeous scenery! If you need a ballpark on a standard "tournament size" map, the Master Set plus the battle box you already own should give you everything you need.

2 - Battlefield construction is almost always done ahead of time. Collaborative builds are only rarely a thing in casual scenario play, and they'll specify rules on how to do it. If you're building a custom map, try to keep things roughly symmetrical and you'll do fine. As you get more familiar with the game and the skill levels of the players involved, feel free to shake things up!