The final report of the BDHS 2011 explains the selection of the biomarkers subsample in page 5:

In addition, in a subsample of one-third of the households, all ever-married
men age 15-54 were selected and interviewed for the male survey. In this subsample, a group of
eligible members were selected to participate in testing of the biomarker component, including blood
pressure measurements, anemia, blood glucose testing, and height and weight measurements. Table 1.2
shows which household members were eligible for which biomarker testing.

The subsamples selected for the biomarkers are detailed in the following table of page 6 of the same final report:

Table 1.2 Eligibility for anthropometric measurements and biomarker testing, 2011 Bangladesh DHS

Third of 18000 households is about 6000 households. The number of women in these 18,000 households will depend on the average women per household. The same goes for the number of men per household. The number of children 6-59 is not 6000/3 it is the number of children 6-59 in the third of the households in the survey. You probably need to check the table 2.7 in the final report to get an idea of the number of children in the households surveyed.