This isn't really feasible within DF's graphics framework.
Think of each tile in DF as a pixel, rather than a grid over an image. While each pixel has many different values it can display (red, green, blue, combinations of each), the pixel itself can't get "bigger"... It's still just a single, physical pixel. You can't get one pixel to cover or overlap its surrounding pixels, unless you're really good at breaking the laws of physics.
What you
can do is get the other surrounding pixels to change what image
they're displaying, in order to give an illusion of size. However, they're bound by the same rules as the first pixel, meaning they have a limited selection of things they can display (while they can also display red, green, or blue lights, they can't display "part of the next-door pixel's red glow", they can only display values centered on themselves.)
So while you can't make one large letter/symbol for a creature, such as this:
S
You
can make a repeated block of the symbol, like this:
SSS
SSS
SSS
Or do it like Incursion does and use a designated "extra size tile", like so:
╬╬╬
╬S╬
╬╬╬
That last option serves to make a visual distinction between "9 S-creatures in formation" and "one really big S-creature"
However, even deciding upon a good solution for the
graphics of a multi-tile creature, there are plenty of other considerations that are difficult to work with... Such as how to handle ramps, traps, having one portion get "flung" by a raising drawbridge, having one portion get atomized by a lowering drawbridge, climbing, doors, attack range, wrestling etc. etc., and that's the main problem standing in the way. Well, that and even just figuring out how to code their movement and pathfinding, heh.
You could theoretically also make large composite symbols using other ASCII characters, like this:
╔══
╚═╗
══╝
That only works for some symbols though, and only on certain size creatures, and also it would probably make any graphics modders hate life.