The GUI won't tell you what 'exactly the same' is until you actually put in something.
You can probably deduce it by adding up the heights that play a factor (top & bottom cell margins, paragraph height, space before and after)
But I am choosing to be less scientific and follow the 'suck it and see' methodology. I found that '0.49 cm' Exactly appears to give the same row height without the ability to get bigger. If you put this measurement in on your machine it will automatically convert to show what that is in inches.
I note that making tiny incremental changes to an exact row height is ignored by Word BUT if you make a big change and then make that tiny change then you will see the tiny change actually sticks. I don't know why Microsoft would have thought that was a good idea.
eg. Change exact row height from 0.48cm to 0.5cm - remains at 0.48cm
Change exact row height from 0.48cm to 0.8cm to 0.5cm - row becomes 0.5cm