Sizing “symbols” inside icons

Question

Let’s say I want to make guidelines for using a plus symbol inside of icons in an icon set.

For example, I have a “New” icon which is a rectangle with a plus inside of it. I also have a “Copy” icon which consists of 2 smaller rectangles in a stacked style with a plus icon in the top right rectangle.

If the plus symbols were the exact same size in both of these cases, it would look “off” because the rectangle:plus ratio is different. How do you come up with dimensions for the plus symbol in this case?

Answer 1

Best if you were to try to visualize these I think. Here I did a quick mock up. Obviously these might not be the same as your icons, but the issues will be similar. Anyway, this is how I might handle the issue you describe.

1. The "New" icon. Note that the width of strokes of both the rectangle and plus symbol are identical.

2. The plus symbol is exactly the same size, but it looks too large inside the smaller rectangle.

3. The plus symbol has shorter arms, but the stroke is still the same thickness. However, when comparing with icon 1, there's a bit of an optical illusion going on which makes it look as if the lines of the plus are slightly thicker.

4. The plus symbol has a very slightly thinner stroke, to compensate for the illusion. I would estimate this stroke thickness change by eye until I see that it looks more visually similar to icon 1.

Answer 2

I would keep the ratio of the plus to the relevant rectangle consistent; this also implies the need for enough padding or whitepace in the plus to rectangle ratio in the "new" icon, which will make that icon more legible anyway.