I frequently have to make overlays to change details (e.g. dates) on signs.

So if the font is Helvetica Bold and an uppercase letter is 172mm high, what font size do I need?

Note that simply calculating 172mm x 25.4 = 6.77 inch x 72 = 487pt is not correct, because that is the height of a single-space line, not the letter height. 487pt would be too small. And the ratio of letter height to line height varies between fonts.

I usually use trial and error, but if anyone knows an online tool, that would be great.

723pt with Myriad for H letter ?

What program are you using? (I'm on Linux, so may not be available!)

https://inkscape.org/