Different printers will have different printing machines. There is no optimal size - the most cost effective way to produce the job should dictate the size of the magazine.
Of course you should get options and price levels you're happy with.
For example, I moved from a 260 wide magazine size to 250mm wide magazine (a mere 10mm) and it saved €10k a year in print costs.
This was due to more pages being allowed to be imposed on the same press sheet, which equals less press sheets per run, which equals less time on the print machine, which equals less time at the guillotine, which equals less time at the packaging, which equals less weight for postage/shipping.
Therefore - your printer will guide on your press size, the allowed margins (as they have to allow for grip on the plate and sheet etc, and have enough trimming on the edge of the sheet that text and images don't stretch).
Rule no1 of starting a new print project - talk to your printers and get their guidance.