One scheme can be to assign to each stratum a (rounded) number of random groups proportional to its (weighted) sample size - with the total number of random groups taken as 100-200 - and then adjust this number it to ensure that it is not less than 2 for any stratum. As noted, grouping of small PSUs within and across strata, and grouping of strata to form fewer and larger computational units is generally desirable to improve stability of the results. Our general recommendation is to begin, as the basis, by defining 100-200 computational PSUs of approximately equal size, and then adjusting and fine-tuning the scheme in accordance with details of the actual design. The final choice is always a matter of statistical judgement and numerical experimentation.