A detailed explanation of the factors which regulate cell size (i.e. the size to which cells grow) has not yet been found. Since (most) cells stop growing when they reach a specific size, it's suggested that there must be some feedback mechanism to restrict growth beyond that point.

The existence of a “size control” is well known and the control has been studied for a long time, but it has been remarkably resistant to molecular analysis. The attainment of a critical size triggers the periodic events of the cycle such as the S period and mitosis. This control acts as a homeostatic effector that maintains a constant “average” cell size at division through successive cycles in a growing culture. It is a vital link coordinating cell growth with periodic events of the cycle. A size control is present in all the systems and appears to operate near the start of S or of mitosis when the cell has reached a critical size, but the molecular mechanism by which size is measured remains both obscure and a challenge."

Source : Growth dung the cell cycle (paywalled)International Review of Cytology, Volume 226, 2003, Pages 165-258

Also see : Cell proliferationplugin-autotooltip__plain plugin-autotooltip_bigCell proliferation

When organic 'tissue' grows (either during embryonic development, post-natal growth, or tissue regeneration) there are two processes in operation which determine the final size of the tissue or organ. The first is the size of the cells themselves - which is governed by an as yet unknown process (see:

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