I've 3 questions:
1) At my size is this current board way too small?
2) If yes, what difference will this make at my level?
3) What size board (I know it depends on the make but as a rough guide) would you suggest for me?
As a beginner snowboarder the shorter board will make turning easier, and not much other differnence at your level, starting shorter will make learning easier for you, good choice
If you progress to riding fast you will want to get a little longer board. I weigh 190 and have a concave 159 cm board for riding fast and a flying V 163 for soft snow which is probably a little big for me.
When you are ready for a new board I'd demo some that are a little longer than what you have.