Using the softkeys on the left side of the screen, select "+Width" from the Horizontal measurements. That will measure the width of a pulse at the "middle" threshold value.
The "middle" threshold values defaults to 50%, i.e. half maximum, which is what you want. But it can be changed via
Measure > Setting > Type > Threshold.
This is described on page 6-26 in my version of the manual. (Figure 6-3 and the text below.)