OK I am a size 20 veteran of many family photoshoots. I always swear I'll lose weight before the next one and never do.
First of all... you are absolutely doing the right thing by having the photoshoot now. It's always tempting to put them off until you feel like you look "better". But I have friends who kept on feeling like they weren't looking "good enough" for a family photoshoot... and ended up never doing one. Now it's a bit late - their kids are at uni and grandparents sadly passed away. The time is now, podgy or not!
Some ideas from my own experience:
Wear black or dark navy or very dark grey, with long sleeves. Plain smart-casual is fine. Not only is this flattering on the larger person, it also makes your face the focal point, which is the whole point of portraits. Nothing fussy, patterned, shiny or with logos. NEVER wear white (I made that mistake a couple of times, never again!).
Trousers are probably best. Very dark coloured smart jeans are fine. Leggings/joggers will not look good. You may be arranged in a group pose where some people are sitting on the ground - this is very awkward in a skirt. Also means you don't have to worry about tights etc.
Plain black or very dark coloured leather boots or shoes. Most trainers can be too noticeable; it depends on their style.
If all this sounds boring... that's because it's meant to be. The ideal portrait outfit is the one you don't really notice when you look at the photo. You just notice the person.
Layers can make you feel more confident and look more polished. So a black blazer or casual jacket over a black T shirt, for example.
Other points:
If you're in a studio, you may be able to pick a dark backdrop. A dark backdrop and a dark outfit work wonders if you're unconfident about your weight.
Similarly, if you have kids, positioning them in front of you can obscure some areas you would rather weren't on display. Or other adults, depending on their height/pose! The people at the back of a photo will always look a little bit smaller just because of perspective.
I hope this doesn't sound patronising. I just want to pass on stuff I wish I'd known to start with.