engine

Well, till a few years ago 4 cylinder configuration was used for engines from 1l to 2.5l (or 3l) - this was almost tradition since it gives the best compromise between fuel economy and NVH. 4-cyl engine has primary balance but suffers from secondary imbalance which causes it to have a sort of up-down vibration (to be correct it is in the direction of the pistons' orientation) which for capacities upto 2.5l can be damped out simply by mounting the engine appropriately. 3-cyl was not used for 1l engines due to it being harder to quieten in terms of NVH (but for lower capacities like 800cc, 3-cyl was used as it was easier to damp it by proper mounts or balance shaft).

However recent trend has been that 3-cyl config is now used even for 1l or 1.2l engines (examples are the k-series, fabia or polo engines). This shows than there is an improvement in materials and technology where a 3-cyl engine can be used for higher capacities.

Most luxury cars with engines bigger than 3l are either V6, V8 or l6 - V6 not as quiet as the V8 or l6 but has the distinct advantage of occupying less space. l6 by its nature has both primary and secondary balance - so it can be made in any capacity desired, and is the quietest configuration. Again depending on space/design constraints manufacturers may go for V12 instead of l6 - basically "V" two smaller l6 to double the capacity yet keep the engine small enough. For example, a 6l l6 would be much bigger (longer, rather) than a 6l V12. Where a V12 would be overkill, a V8 may be used. It should be noted that V12 has the same desirable NVH as l6 (it is basically 2 l6's). V8 though not as quiet as l6 or V12 is still a vast improvement over odd-cyl or 4-cyl configurations in terms of NVH.