There are too many variables to predict plant size via age (days in dirt). Sex, strain, lighting, feeding and environmentals ( RH, temp., etc...) are just a few of the minor factors affecting early growth size. Single largest determinant to your question is the size and shape, along with timing, of the pots you use. I can, and have, taken clone-sisters and varied their growth rates by 100%. Means I can have one plant at 8 inches when sister is 16 inches.
I've found that I get the most efficient growth when I transplant seedlings twice. From a solo cup to a 7 inch square intermediate pot until it reaches 14-16 inches then they go into a finishing 3 gallon pot. I grow mostly bushy indicas so others may have varying results than mine. With my strains I leave them in the solo cup, braced, until they are 100-125% as tall as the solo cup. Then they go into the 7 inch berry pots. They go from the veg room (400W) to the flower room (1000W) when they reach 20-24 inches. My lighting is set up so that I want my flowering plants to finish around 48 inches tall. My indicas just about double in height when switched at 20-24 inches. To do it right you need to know your strains.
Good luck, BigSteve with perpetual grow since '09.