Fonts can be referenced by Pixels (px) or Points (pt) and have different sizes. These tools will help in comparing or converting between the two.

reeddesign.co.uk/test/points-pixels.html
Here's a chart that converts points to pixels (and ems and %). It's an approximation, which will depend on font, browser and OS, but it's a good starting point.

websemantics.co.uk/resources/font_size_conversion_chart/
A chart of font sizes stated in points, pixels, ems, percent and keywords. With example text in serif, sans-serif and monospace type.

www.sherriwyche.com/tools/fontconversion.html
This is a chart for Font Size Conversions from PT to EM to PX to EX to CM to IN to MM to PC to %.
These font size conversions are all approximate but at least give you a quick look to see the general size conversion. You can then tweak it more ...

www.endmemo.com/sconvert/pixelpoint.php
Topography unit conversion between pixel and point, point to pixel conversion in batch, px Point conversion chart

px-em is a px to em tool which allows you to work out the em sizes from px. Just add the parent px size in the top box, required size in the bottom and you're done!

One-click PX/EM/%/PT conversion tables, a custom conversion calculator, gnd generated reset CSS.

www.trishasdesignstudio.com/font-size-conversion-chart.asp
This small site has some very useful pages: Web-Safe Fonts, Font Sizes, Color Charts and HTML Lines.