How much is one em (image size unit)?

[Akib Azmain Turja]

I'm trying to show an image such that it has same height as the text and
resizes relatively with text scale.  I've achieved this with the 'em'
unit as described in (elisp)Image Descriptors.  But 1 em seems to be
pretty smaller than the font size.  How to figure out correct value in
em unit such that image and the text has correct height?

So far I've reached till this:

--8<---------------cut here---------------start------------->8---
(insert (propertize "f" 'display '(image :type xpm :height (1 . em) :ascent 
center :data "/* XPM */
static char * blarg_xpm[] = {
\"16 7 2 1\",
\"* c #000000\",
\". c #ffffff\",
\"**..*...........\",
\"*.*.*...........\",
\"**..*..**.**..**\",
\"*.*.*.*.*.*..*.*\",
\"**..*..**.*...**\",
\"...............*\",
\".............**.\"
};")))
--8<---------------cut here---------------end--------------->8---

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