The table size is the sum of pagesize*(number of table pages) plus pagesize*(number of index pages)

Table Size

 db2 "with rs as (select distinct(substr(t.tabname,1,35)) as table, t.card as card, \
t.type as type , t.fpages as fpages, case when ind.nleaf is null then 0 else \
ind.nleaf end as nleaf, ts.pagesize as pagesize,  t.tabname tab2, t.tabschema as schema \
from syscat.tables t inner join syscat.tablespaces ts on t.tbspaceid=ts.tbspaceid \
left join syscat.indexes ind on t.tabname=ind.tabname) select table,\
(sum(nleaf)+fpages)*pagesize as size,card from rs where schema='<schema_name>' and \
type='T' and table='<table_name>' group by card,table,pagesize,fpages"

Tables Size for one specific schema

db2 "with rs as (select distinct(substr(t.tabname,1,35)) as table, t.card as card, t.type as type , \
t.fpages as fpages, case when ind.nleaf is null then 0 else ind.nleaf end as nleaf, \
ts.pagesize as pagesize,  t.tabname tab2, t.tabschema as schema from \
syscat.tables t inner join syscat.tablespaces ts on t.tbspaceid=ts.tbspaceid left \
join syscat.indexes ind on t.tabname=ind.tabname) select table,(sum(nleaf)+fpages)*pagesize as size,\
card from rs where schema='<schema_name>' and type='T' group by card,table,pagesize,fpages"

Schema Size

db2 "select ((select sum(fpages*pagesize) from syscat.tablespaces as a, syscat.tables as b where \
a.TBSPACEID=b.TBSPACEID and tabschema='<schema_name>' group by tabschema)+(select sum(nleaf*pagesize) \
from syscat.tablespaces as a, syscat.indexes as b where a.TBSPACEID=b.TBSPACEID and \
tabschema='<schema_name>' group by tabschema))/1024 as SCHEMA_SIZE_KB from sysibm.sysdummy1"

Database Size

db2 "select sum(TBSP_USED_SIZE_KB) as DATABASE_SIZE from sysibmadm.TBSP_UTILIZATION"