Hey guys

This will be my first SUP and I'm seeking some advice as to where to start. I'll obviously demo some boards and will speak to some pro's in store, but am keen to hear if anything stands out as to a good starting point. I'm about 6' and weigh 80kg. I'm looking for something to use in both the flat (more often) and probably up to 3-4'. My surfing skills are intermediate. From what I've read so far, something around the 10'x30''x4.25" looks about right, but is this too big/small for a first board? Do litres also matter? I'll also throw my young girls on the front from time to time (in the flat), but this isn't the key criteria. Not looking for a boat, but something stable in both flat/waves. An open question I know, but any thoughts as a guide to where to start would be appreciated.

Cheers