How we can assure the minimal inconsistency within the WFE servers?
By maintaining the smallest refresh time that is reasonable
What will be the best way to estimate the output cache refresh? Any tips on it?
It is something that you will have to just figure out by balancing the performance gain with the inconsistency between WFE. I would recommend start with 45 seconds and see if that gives the performance boost you are looking for without a noticeable inconsistency and then adjust accordingly http://technet.microsoft.com/en-us/library/cc261797(v=office.12).aspx .
Speaking about the Object Cache, any ways to determinate a recommended max size for it?
Recommend at least 500KB per Site Collection, and I would add another 500KB afterwards just as a buffer (You know there will be growth and you don’t want to have to adjust to soon).
Remember that we will be allowing SharePoint to hold this in-memory so we will possibly have to add RAM to each WFE account for the extra growth.