They are likely doing one of these options:
- using different objects,
- having the same object but slowly raising it up, so more of it is visible every growth cycle,
I doubt they are resizing the object as that would be a lot more complicated albeit could be quite prettier.
I don't think you will find a tutorial specifically for this but you can take a look at other scripts that grow drugs and see how they do it, maybe adapt some of their ideas in your own way.
I recon you could do it similarly like this: player plants weed -> object is created, timer is started -> every timer is a single growth cycle, so after it's finished, the old object is removed; a new, bigger object is created, the state and data of the plant are updated and a new timer is started or the same one is looped -> player harvests the plant, timer is killed, player gets everything and the plant is deleted from the server.
Something like this, I guess.
Here is some filterscript I found
https://sampforum.blast.hk/showthread.php?tid=609790