There are multiple approaches that can be used.
But wouldn't it be simpler to select data from the portal row just by clicking either the portal row or a button placed in the portal row instead of selecting from a drop down list?
That also makes for a very simple script.
Once you have selected a portal row by either drop down or mouse click, it's not clear what you want to do with the measurement and units?
Are you putting that data in a field, several fields? In a new record? or ???
Those details can make a bit of a difference in the best approach for what you want to do.