It's not possible to do it without touching the code a little bit. Good news is that it's very simple.
Lets say you want the icons 50% larger (48x48 pixels instead of the default 32x32) and don't mind that the enlarged icons will not be as smooth:
In chart1.cpp (line 1586 in the current
2.6 beta code from git):
in bitmaps/icons.cpp (in function initialize_images()):
Change all lines like
_img_WHATEVER= new wxBitmap(wxImage(sm));
_img_WHATEVER = new wxBitmap(wxImage(sm).Scale(48,48));
If you want a different size for the icons, just use other number instead of 48...
Then rebuild opencpn
Or you can regenerate the icons from the SVG sources to be the size you need and look pretty - If you are on Linux
and have Inkscape and perl, read the create_all.sh script in src/bitmaps
Hope it's clear