For whatever reason, some people are concerned with imbalance when analyzing data from randomized experiments. The concern may be more general, but its fixes devolve into reducing imbalance on observables. Such fixes may fix things or break things. More generally, it is important to keep in mind what one experiment can show. If randomization is done properly, and other assumptions hold, the most common estimator of experiment effects – difference in means – is unbiased. We also have a good idea of how often the true estimate will be in the bounds. For tightening those bounds, relying on sample size is the way to go. General rules apply. Larger is better. But some refinements to that general rule. When everyone/thing is the same – for instance, neutrons* (in most circumstances) – and if measurement error isn’t a concern, samples of 1 will do just fine. The point holds for a potentially easier to obtain case than everybody/thing being same – when treatment effect is constant across things/people. When everyone is different w.r.t treatment effect, randomization won’t help. Though one can always try to quantify the difference. More generally, sample size required for a particular level of balance is greater, greater the heterogeneity. Stratified random assignment (or blocking) will help. This isn’t to say that raw difference estimator will be biased. It won’t. Just that variance will be higher.
* Based on discussion in Gerber and Green on why randomization is often not necessary in physics.