How to define Project Concept

[Sergio Rojas]

Hi guys, currently I am working full time on developing a mixed (commercial and residential) building located on a nice property down-town in San José, Costa Rica that I owned. I plan to sell some units and rent the rest (about 50-50).

I have designed several projects for clients but never something like this, newbie here. 

Right now I am on an early stage of defining the Concept or Basic Design, I have no problem at all with the arquitectural and engineering side of the Design but I have absolutely no clue about how to define the number of rooms, size of each unit, final user market sector and related subjects, so I would love to hear from a savvy developer about some general rules of thumb and methods to approach this issue. 

I know there is nothing worse than a general question but at this point I am really interested in just listening to your advice and general guidance, or reference to a nice web site or book about it. I will get more specific in time, you´ll see...

Thanks in advance.