On a steam locomotive, the grate area is one of the more important dimensions. It's the size of the grates at the bottom of the firebox, and therefore the area that the fire covers. For a given fuel, the grate area determines the maximum amount of energy that can be put into the steam engine's system - that amount is called the grate limit. Only so much fuel can be burned in a given amount of time in a firebox, no matter how much fuel is delivered and how fierce the draft through the firebed. Furthermore, if the draft is too fierce, the phenomenon of fire lifting occurs - the draft is enough to blow the coal right off the grates and down the tubes and flues.
Since increasing the energy put into the system is the easiest way to produce a more powerful steam locomotive, it's hardly surprising that grate areas grew to stupendous proportions in later locomotives. The largest firebox ever constructed had a grate area of 182 square feet (16.9 m2) and was fitted to the Northern Pacific's class Z-5 Yellowstones (2-8-8-4s). The manufacturer, Alco, served dinner for twelve in the firebox to celebrate the completion; there was ample room. These locomotives burned low-energy lignite coal and needed vast grates to do so.
More normal for the giants of steam in the last days in the United States were grates of between 120 and 140 square feet (11.1 and 13.0 m2) which were fitted to the Union Pacific's Big Boys, the C&O's 2-6-6-6 Alleghenys. The largest American passenger type in general use, the 4-8-4, averaged around 100 square feet of grate area, while locomotives with a two wheel trailing truck maxed out at just over 70 square feet.
Locomotives in other nations were rather smaller. In the United Kingdom, the Great Western Railway's famous Castles had a grate area of under thirty square feet while the LNER's famous A4s had just over 40, and the LMS' Stanier-designed Coronation class had grate areas of fifty square feet. One of the reasons was that grate areas even that size were impossible to fire to anywhere approaching their grate limits with hand firing; automated stokers were fitted to few locomotives outside of North America.
A few statistics and stuff from Wes Barris' excellent steamlocomotive.com