How far is 1 meter?

Solution 1:

On the Arcane Statistics screen (bottom of the left character information screens, press J by default), you can see the Adjusted Range of your currently equipped weapon(s) once you have expanded the Right/Left Hand Weapon section.

Here is the display for one of my Embermage's wands (which apparently has a range of 7.2m):

Using this information you should easily be able to work out how much that +0.67m range improvement has actually extended the range of a specific weapon by, which is what I assume you really want to know here.

You can also get a good visual ideal of how distance translates on screen by firing a weapon of known range from a standing position and seeing how far across the screen it's shots go. Of course, this assumes the "adjusted" range is what it will actually fire at the moment.

And, if we really must some maths:
Using the wand above, camera fully zoomed out, at a resolution of 1920×1080, with screenshots taken as my shots are fading away at their full range, and using some crude approximate pixel measurements taken with Paint in Win 7, I reckon I'm seeing a rectangle of space approximately 24m×16m (completely ignoring the 3D perspective, etc) in size, which represents ~12m horizontally and ~8m vertically to the edges of the screen from the player character (in my case at least).