Any plant can be cloned at any stage but it is easiest to take it from a plant in vegetative state as cuttings from flowering plants take a good bit more time to root .
The size of the clone when you put it into flowering is totally up to how you want to grow . Sea of green - you put them in a few days - a week or two after they root . Big plants , you veg em for a month give or take depending on the strain .
It isn't like growing from a seed since the clone is as old as the mother so it is old enough to reach full potency no matter the size . If you planted the seed 2 years ago then the mother and every cutting is 2 years old as well genetically . You can clone clones of clones - it doesn't matter as long as they are healthy .
There are many ways to clone so read up on a few and pick a method .
If you are taking cuttings pre-flowering make sure you label all the cuttings and the mothers so you know what you are working with . That way when you harvest you can pick which clones to make your mothers for future cloning based on yield , potency , flavor , flowering time , etc .
Again , make sure you have a Sharpy and label EVERYTHING !!!
Don't forget , read , read , and read some more . Then you can pick a method best for you or try a couple of them back to back since certain plants respond better to different techniques .
Hope this made sense , been a long 4th of July...
Good luck .