When VOCALOID3 Editor is launched, it automatically creates a part of 32 measures by the default, and it can be modified to any length, anytime.

When you point out the right end above the track editor (which is colored in light green) with the arrow mode (right click on the track editor and choose [arrow] from the menu), the arrow cursor will turn red.

Then, while the cursor is red, drag the cursor horizontally to adjust the length of the measure.

If you wish to extend the measure length, drag the cursor to the right.