I'm looking at replacing most of the components in a 4-5 year old custom build I have. My current mobo is a Gigabyte GA-H55M-UD2H which is a micro-atx, I don't necessarily want to go micro-atx again so that I have more room for future upgrades but I'm concerned about what will fit comfortably with my GPU etc.
Ideally I was going to use a regular ATX form factor mobo but I'm worried it won't be a good fit.
The bulk of motherboards out there are micro-ATX and standard-ATX. Those will fit in just about any standard ATX case. Tons of mid-tower cases also take boards larger than standard ATX.
Unless you plan to use one of the more exotic Wide/Extended ATX form factor boards in your case, you should not need to worry about whether or not a standard ATX board will fit in your mid-tower unless your mid-tower is a weird non-standard contraption.
When in doubt, check the manufacturer's website if you no longer have the manual.