Specify default size of Linux desktop icons

Specify default size of Linux desktop icons

I only recently made the switch to Linux (great OS, btw. I very much enjoy
the shell. Way more flexible than anything I'm used to) but am already at
the point that I'd like to deploy a small app I designed over the last two
weeks.
Thing is, I want it to look good. I want it to be a complete experience,
so everything should look and feel great from the start.
I really enjoy the look and feel of Amiga'esque/MacOSX'esque icons and
would like to try something similar. i. e. the original size of the icon,
when my app is first installed, should be much larger than the default
size of most other app icons.
I know there must be a way to do this, since Steam's default icon is
almost three times as large as all the other icons on my desktop.
Question is, how can I achieve this? Is there something I can put into the
desktop entry? Or perhaps someone would be able to help me locate the
Steam icon file (it's a png, as far as I can tell)? "Whereis steam" yields
nothing of the sort unfortunately, and the only Steam icon inside
/usr/share/pixmaps is the Windows/Wine Steam one.
PS: I'd like to use .svg for my icon, if that matters (I assume it's less
obvious to set a default resolution for a scalable image format.
Hopefully, I'm proven wrong though).
EDIT: I found an icon inside /usr/share/icons/hicolor/256x256/apps.
Unfortunately, it's lower res and will still always revert to what looks
like 32x32.