A "bug toe" is simply an expression for a very small amount.
(I haven't heard this expression used for years but it may be more common elsewhere. Where I live, it's more common to hear a very small amount referred to as "a whisker")
If you want a quantitative value you might consider that:
(1) a human hair is about
(2) a human hair is visible, a bug's toe (unless you have better eye sight than I) is not.
that might suggest that the width of a bug toe is no more that something of the order