The primary motivation for the development of large capacity launch vehicles seems to be that of efficiency - a bigger vehicle will be more efficient at delivering a given payload to LEO than multiple smaller vehicles.
However, there seems to be a number of difficulties in scaling up. Factors like chamber pressure and fuel chemistry seem to place a hard limit on the amount of thrust and ISP possible for a given size of engine. Engines only seem to get so big; one factor is combustion instability. Bigger launchers then seem to require larger numbers of engines to achieve a given total thrust. For example, the proposed SpaceX ITS booster would have 42 Raptor engines. That raises questions of reliability; more things that can go wrong increases the prospect that something may go wrong.
My question is: do issues associated with scaling up ultimately place a practical limit on the maximum payload to LEO for a single vehicle launch? Is it a hard limit or one which could increase over time with the further evolution of space technology? What is the current practical limit? What could be the limit in 10 years, 20 years, or 50 years?