How do we know bulkiness of a substituent in organic reactions? Which is more bulkier among CH3, OCH3, NO2, Cl, OH etc. ?

Steric factor is important in organic chemistry. The bulkiness depends on the number of side atoms and type of side atom. It also depend on atomic size.

The ones having higher number of substitutions like isopropyl, or tert-butyl groups are considered bulky because they are larger in size and can produce some severe steric effects in the reactions. On the other hand simple straight chains like ethyl or propyl are not that bulky in comparison.

Eg. NO₂