Measuring the volume of a cell is often done by labelling the cell membrane or its cytoplasm. Analysing large flat cells this way is easy but it is much harder for tiny cells like blood cells, yeast or bacteria.
Another way to measure volumes is to use a negative stain, i.e. where the medium is made fluorescent with a dye that does not go into the cell. The cell appears as a black hole in fluorescent images and unlike lipid-based membrane labelling, borders are even and easy to segment.
While many dyes can be used for live cells, one must choose large dyes when negatively imaging cells that have been fixed and permeabilized.
This paper and this one use high molecular weight (2000 KDa) Dextran to achieve these results and measure the size of bacteria.
This recent paper optimizes the technique.