we are planning to create one common vcenter for some use case
My question: based on current assessment, we came to deploy it as small size, lets assume down the road, if we need to change VC size from small to large is it possible?
Yes, it's possible
while you decide the size of the vCenter, take a look also at Storage Requirements:
Well, I think the sizing options and drop downs [tiny, small, medium, large and so on] are shown only initially, also this setting will not save or cached in any of config file.
but if I am not wrong if you are upgrading VC using ISO image you will see the same screen again and can able to change the size settings as per your growth
You can only see the screen in your image if you update the vCenter. If instead you want to increase the size of the vCenter you have to follow the second link 'Resize down the road"
Update vcenter screens:
enter the dimensions you prefer (this is an example):
Resize down the road: