It isn't one value, but the UPM or em-size. The standard for OpenType fonts is for 1000 units, while in TrueType fonts, the UPM is by convention a power of two, generally set to 1024 or 2048. However the number you choose is up to you, and put simply, this value can be defined as the space that spans from the lowest-possible descender to highest possible ascender.
I say lowest and highest possible because while each glyph is different, the values must be the same for the whole font, and also across all styles in the family. This is not to say that the outlines for any glyph will occupy this entire space, but rather these are the dimensions that all glyphs will fit into.
The descended looking cursor pictured in your post could also be caused by setting an underline height and width that extends below the descender. Less likely, though still a possibility, is that your glyphs may not be sitting on the baseline. I have never used "Type 3.2" but it is hard to imagine the latter could be the case without you being aware of it.
A similar concept which you should familiarise yourself with is how the x-height affects the size-variance between different fonts set at the same point size. More on that: