I enjoyed learning foiling last summer, using a Moses Vento 790 on a 115 L Fanatic Blast. I don't have cambered race or foil sails and weigh 64 kg. One thing I've come to realize is that I think this board is too small for light wind foiling. Im not entirely sure and I'll keep working on technique. I was, and still am, hoping to use one board for 10 to 22 knots.
I may start saving up and looking for a foil board that would be better suited for light winds, particularly if I ever come across a good deal. Any suggestions on minimum width or volume? I don't intend on using a sail larger than 6.5.
Width is much more about controlling a bigger sail.
If I were you I'd demo the 1100 wing and see where that puts you rather than a bigger board.
At ~90kg I'm on a 122L board and 1700cm2 foil with max sail size of 6.3m, anything above 12kts and I'm foiling. I think with a full on race set-up I can probably do 8kts but it's not worth the added size and complexity.
Yeah, right when I bought the 790 from Sailworks a few guys from Vancouver bought the 1100. Made me wonder if I had made the right decision. Sailworks crew never suggested the 1100 to me. I wonder what I could sell the 790 for, I can't see me needing two wings.
For your weight i see something like the severne alien 115 and an additional frontwing, moses 720, for more wind. The 790 is great. Do not sell it 
I don't know if going bigger would benefit you much with a 6.5 max but a foil shaped board would. I agree with Patk that something with more volume and width in the tail will make light air, upwind and take offs easier without becoming unruly as the wind picks up. Blasts are nice boards but most dual purpose boards are pinched more in the tail (makes jibes easier) and that costs a bit on the foiling side.
There is room in your quiver for more than one wing. If you go 1100, you'll want to keep the 790 for bigger wind days and then, again, as PatK, suggests, look for a higher wind/ faster wing like the 720 as time and your budget permit. The 1100 is like a sail size, at least, improvement over the 790 in light air if that's what you are looking for.
You just need more time on the water. I'm 79kg at the moment, and on the Alien 115, with a 1000cm2 front wing, I'm good from <10kts to 25kts+ with a 6.0, 4.8 and 4.0 sails.
Board with a wider tail would be nice, but not super critical especially if you are only planning on using smaller sails.
Technique makes a massive difference foiling, for a rough benchmark, with some practise you should be able to foil before it's windy enough to waterstart.
