why solar cell size is always 156*156 mm ? why not 200*200 mm?

Originally, solar cells where cut from ingots used to make integrated circuits. And I.C. production costs are driven down the larger the ingot slice is (more circuits on one slice, same processing costs/time).

Now--I would guess that there are dedicated silicon wafer production facilities that are not directly tied to the IC market--But they still have practical limits on how the can grow the raw ingot--And cutting into thin wafers for solar cell production (waste of saw cut, thinner wafers are easier to break during manufacturing/use of solar panels, etc.).

There are alternatives. Evergreen Solar (may they rest in piece) had a "ribbon" process where they could grow flat sheets of silicon crystals then just saw them to length (I believe a Chinese solar panel company purchased the process patents and equipment from Evergreen's bankruptcy).

Then you get into other questions... A silicon solar cell is a just a (current mode/source) battery. If you could make a 1 meter x 1 meter square cell cost effectively, then mount it to a glass panel--You would have a 0.5 volt @ 150 Watts -- Or Vmp~300 Amps.

You can figure out how much copper you would need to move 300 Amps with less than 0.005 volt drop (1% voltage drop). Does not make much sense to design a cell/panel this way.

You are better off designing 60 156mmm square cells in series and putting them into a panel with Vmp~30 volts and ~5 amps. You need a small fraction of the amount of copper to move the same 150 Watts of power--And it is already closer to a "useful" working voltage (10 panels * 30 volts Vmp = 300 Volt string voltage--Vs 600x 0.5 volt 150 Watt panels to get 300 VDC working string voltage).

Of course, you can design larger cell panels--But that then forces the panel larger (60% larger square meter, 60% heavier, 60% more power for 200mm vs 156mm cells). And the larger panel will possibly need thicker glass and heavier frame to handle wind/snow loading (more or less, 2x larger mechanical "thing" needs to be 4x stronger because to keep internal stresses from bending frames and breaking glass...). And you need a source with larger glass for your panels, larger processing ovens for your cells, a source of cost effective 200mm cells (all of which won't happen until somebody is actually willing to buy all of that "up sized" raw product--A bit of which came first, the chicken or the egg question).

And that larger panel/cell with 60% more copper wire diameter. Need new/higher current MC4 connectors, etc... Not saying it cannot be done--You just need a market that can use your larger cell/panels and the support equipment that can use those panels...

-Bill