Welcome to the 405th Chris! Deciding which set of armor to make first can be tricky. The super-soldiers of Halo may be built like a 1980's Arnold Schwarzenegger, but that doesn't mean big guys like yourself can't participate! Pretty much every member of the 405th utilizes a 3D modeling software called Armorsmith. Armorsmith is a CAD platform that allows you to modify the size of armor files to fit your body. You start by molding a mannequin with your body's measurements, and then when you import armor files and attach them to the mannequin, you can edit the scale of those 3D objects to fit your body perfectly! TurboCharizard
made a Beginners Guide
that will run you through all the basics.
You described yourself as wide, so for recommended armor kits, I would go with either an ODST or a Reach Spartan. The ODST traditionally has a torso piece somewhat similar to a medieval chest plate that wraps around your entire body, and then the armor parts just get slapped on top of it. The Reach Spartan is similar with a larger and more detailed body piece which you can attach armor and equipment to. (If you can't decide which, you could totally slap ODST armor onto a Spartan suit!) Either way, if you're concerned with the size of your Torso/Abdomen, a Halo 3/Reach body glove would be much more ideal than a Halo 4/5 body harness.
Best of luck to ya!