To the original question from slowboard, as these extremely different answers apparently comes from different riders using different gear and having different "angles", my take is:
Does board volume matter for water starts?
Normally, with medium or big wings like used by most today, NO, volume is only a downside, no doubt
So the old saying that bigger boards (area and volume) are better when you are a new rider, is somewhat an old myth that mostly made sense when only smaller max medium wings were available.
Yes, you can start a tad earlier and learn to make surface transitions easier, with a bigger board when you are a beginner - but after that, no advantages apart from self rescues.
If you learn strapless, volume is a downside also, too difficult.
So in typical foiling conditions, using bigger wings, board volume does not matter, I agree fully
For racewings, volume is a necessity when in the low end, as you need to get some glide speed before you can foil on fast small low cambered wings.
In the utmost marginal winds, volume matters a lot, for all kinds of riders and wings.
Then we have the situations with any kite and wind, where the wind lulls a lot, here volume helps in order to avoid a full stop, and to get up foiling again, thus when riding in the lower end (not marginal) and foreseeing the wind might often both drop for a while (or pick up), choosing a tad bigger board will help you foil easier from the surface in these lulls, and you can usually waterstart to the surface a bit earlier also, and last but not least, glide to shore a lot faster and for sure have a much better chance to get up foiling again.