Hi Rob -b Welcome
,you are pretty much on the mark about the width of the board it is heading towards the SUP width. IMO The super magnum are wave barges design to help the most hapless get their waves, they are also super durable however if in being overweight you have a large chest then I see real discomfort and ineffective paddling. I'm thinking something between 22 and 24 inch would be better.
My thoughts on paddling are that you should be able to pull your arms straight back from their entry point in the water only slightly wider than your shoulder width if you have to flip the arms outward to much in your stroke you can cause serious repetitive shoulder and elbow strain.
As most BIC sellers have a trial board, give one a go and see if it fits and that applies to other boards too,.
I'd be seriously looking at something epoxy, have a look at Global Surf Industries site they have volume and weight recommendations for all their boards.
A simple thought, the better you surf the more you can choose towards the smaller volumes of board for your weight!
Again a lot of GSI Dealers should have some trial boards.
Your height, weight and age ( I know that seems a little personal) will help us advise you on your board choice.
BTW other exercise to help you lose weight and gain mobility etc will help your surfing more , as a beginner it is challenging to get enough water time to take the weight off.
Hope this helps
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun!