Alright. I dried out my empty HDPE vinegar jug, weighed it on my gram scale (3.77oz), and carefully filled it until it weighed a total of 137.21oz on my bigger digital scale. Then waited for it to settle, marked with a sharpie, and used that as a reference to graduate one of the new carboys.
And it turns out that they just barely hold 5 gallons. Like an inch or so from the neck.
So i have two beautiful blue glass 5 gallon secondaries. That works.