I think OP is asking who makes the largest Insert?
@Kain Are you looking for new or used?
First size the appliance for the square footage being heated in your heating zone.
The Insert or stove is also sized to the chimney.
A larger fireplace opening can be closed off with a faceplate if the opening is too large for the correct size Insert. Don’t buy an insert to fit the fireplace, buy it for the correct heat output.
Next decide on stove type. Older or newer EPA Certified that burns nearly smokeless and much more efficient. They are; Secondary burn or catalytic types.
The secondary burn type won’t close down as far, so it has more heat output on low for a shorter duration than catalytic. Secondary burn type has more visible flame on lower setting.
Catalytic types burn smoke hotter in a much smaller area than the entire firebox, so have overall lower output turned down low, but continue to burn clean. Where you are you may not need medium to high output constantly, so catalytic is the output you may need.
The only benefits from older stoves is they are more forgiving using less than optimum fuel, and many have a radiant front that sticks out of the fireplace with a cooktop. They eat wood, so should only be installed when rarely used, not as a primary heat source, or when installed for emergency heating and cooking only.
When we know if you’re looking specifically for a Fisher, this is the Forum for answers. If you’re looking for an older Insert, we can move this thread to the Classic Forum for more views. If you’re looking for a newer EPA Insert, we can move this thread over to The Hearth Room where the high tech burners can answer more questions on the newfangled types than I need to know.