There are standards for spools/bobbins/reels
Looking for standards for spools, I started at Alibaba and then came to an actual European manufacturer or retailer of reels: Häfner. They manufacture reels for wires, which are also used for 3D printers. They helpfully provide a chart of their products from about 250 cm³ to a whooping 98454 cm³ - which is about 250 grams of ABS to in theory 98 kilos. However, typically the largest spool you will find for privates is the 300/20 K type with about 3800 cm³, which is more than enough to get a 4-kilo reel of PLA onto it. At times you might find something in the order of 8000 to 10000 cm³ for a rather heavy 8-kilo spool, though that is industrial sizing.
As this single offerer shows, spools are very varied, and the hub diameter of them is not standardized, but the boreholes only come in very distinct sizes.
However, some of the items that are sold as 3D filament spools are actually welding wire spools, for which standards exist, such as this dictating "2‐1/32 inch" arbor holes.
Scale of spools is dependant on the market
Typical enclosed consumer FDM machines are small.
But spools below 1 kilo are not useful for mass-production of large parts and even then, 3 kilos is a sweetspot for handling between ease of moving and time spent changing spools. So industrial machines usually take larger spools - or propriatory cannisters/cartridges with a spool.
One of the big ones is the Stratasys F900. It has a print-volume of 914 x 610 x 914 mm and takes "up to 500 cubic inch" cartridges, apparently the largest size of FDM Filament cartridge offered by Stratasys. For the record: 500 cubic inch weigh about 8 kilos in ABS, and about 10 kilos in PLA.
Modification of existing printers
It is almost trivial to modify an existing printer that takes non-proprietary spools to allow taking in filament from the outside without keeping the door closed. A piece of PTFE tube can easily take the position of the spool inside the machine to guide the filament into the machine-mounted intake. The modification might only need a single small hole in the door for the tube or its fitting to get into the machine.
This way you mount pretty much an adapter for larger spools, but you bypass for example automatic filament detection with the spool unless you also open the door and slot in a "disk" of sorts that contains the RFID with the configuration of the mounted spool.
A random example setup, mounting the "outer intake" in an angled block at the center of the former door: