The short answer is yes, there's a size or quantity limit, but in the interest of not just throwing some arbitrary (and uninformed) standard at you and demanding you conform to it, the longer answer is "use your reasonable judgment"! For example, in theory he could summon the room-sized wurm...for about three seconds, before the exertion of trying to maintain the summon knocks him cold and the wurm disappears. >>

Basically, if there's something in particular you'd like to ask us specifically about, feel free to do so, but if you're just looking for a general rule of thumb: the more things you summon, the quicker you're going to burn out of energy trying to maintain them, and the bigger they are, the harder you'll have to work to maintain them. Thus, you'll be able to maintain one wolf a lot longer than twelve elephants! We're just asking that players be fair and reasonable in their own assessments of how quickly it'd wear their characters out to do what they want to do.