Meristem size is a very controled, highly genetic feature. It doesn't change between big or small branch/root (The SAM and RAM are obviously different).
There are very levels of potency in the meristematic tissue, so you should explain more which section of the meristem is of interest.
SAM (shoot apical meristem)
In *Arabidopsis thaliana* there are 9 long-lived meristematic cells, 3 in each lineage (L1, L2, L3) (https://www.sciencedirect.com/science/article/pii/S0012160609013840)
But the whole meristem is somewhat larger. If you consider the organizing center and cells that react to wushell by creating CLV1, CLV3 you get a few dozens.
If you consider cells that express miR156, miR172 or AP2-like genes you can get a few hundereds.
The scale is similar between species but the number and genes used to asses it vary.
RAM (root apical meristem)
I know less about the RAM but we can consider there the Quiescent center and also the Intial cells togheter called the promeristem.
THe Quiescent center consists of 10-20 cells and there is one (sometimes more) initial per file of cells in the root giving 20-30 cells in the promeristem.
The meristematic zone is somewhat bigger than the promeristem but the cells are pluripotent with relative small mobility.
The most meristematic zone of each meristeme has no more than 50 cells.
The full area considered a meristeme can have a few hundered cells, depending on the research goal.