Circle and Boundary are "areas", ill-defined as they may be, and there are some more exotic targets that have been explicitely bounded in size (e.g. the Road and Road Network Target of the Neo-mercurians that affect everything on a road or road network ... within certain bounds).
But indeed, there is no 15' by 15' square area target. However a Formulaic ([i]not[/i) spontaneous) can use non-standard parameters. It is generally considered "somewhat more difficult than the equivalent target". So if you want your spell to affect an area that is about the size of a standard Room, your spell will end up be somewhat higher level than one that uses a standard Room target. How much harder is left to the appreciation of the Storyguide, and need not actually be harder, nor be a whole number of magnitudes (though "+1 magnitude" is more or less the default choice).
Remember that this is not possible for spontaneous magic which must use standard Hermetic guidelines - or guidelines otherwise explicitly made available to the magus, from things like Faerie Magic, initiations or breakthroughs. If you want your magus to be able to use "15' by 15' square area" in spontaneous spells, he must perform Original Research and achieve a Minor Breakthrough. Then he, anyone apprenticed under him or anyone learning a formulaic spell he invented using that new target will be able to use it effectively as a new standard target.
Without a well-defined boundary (Circle, Room, Structure, Boundary) you can't arbitrarily place your spell and affect a bunch of individuals in that area (e.g. mind-control all humans within 25 feet of that tree). Unless its with something like a single, large fireball, or digging a pit of a given size, or turning part of a lake into ice, in which case you rely on the standard individual size for each form and tack on magnitudes if you need it to be larger.
In most cases, and unlike in That Other Game, you don't usually try to place your spell "just so" to affect a maximum number of individual targets - when you do, it's because you're really affecting a single, smaller or larger, individual and using it to do stuff to the hapless victims in the area (and make a Finesse roll to see if you place that individual correctly). If you want to mind-control a bunch of humans, it's either a Group, an explictly-bounded spell (Circle, Room...), an exotic Target (Road Network, Body of Water...), or a special target in a formulaic spell.
It probably won't break anything, there are way more outrageous targets, but you still need to decide what it means exactly.
If I cast a CrTe to create, say, diamond. Does that mean it creates a single diamond of that size? That it fills it with individual-sized diamonds? Keep in mind that the standard individual for precious gems is much, much smaller than it is for loose dirt.
If I cast a MuAn spell with that target to turn animals green, does it affect every individual animal in the area? What about animals that don't fit entirely?
That's the hard part about specifying a new R/D/T: decide what it means.
Don't forget that Group itself can still affect a single individual. It does cost one more magnitude than the equivalent size-boosted Individual target, but is much more flexible. And you can typically fit more than 10 Individual mass-equivalent of most forms (exotic solid and liquids being notable exceptions) in a 2-pace diameter sphere.
It may get icky when dealing with spirits: how many angels can dance on the head of a pin?
Minor at best, or possibly available from the right sources.
Want to try me in French?