Hello, Leskala,
Judging from the taskbar code, 16x16 is hard-coded there (a bad idea IMO). If it were not,
you could change the icon size using the "Small Icon" option in the Icons
sections of TDE Control Centre (probably a bad idea too).
You should consider opening a bug here:
https://mirror.git.trinitydesktop.org/gitea/TDE/tdebase/issues
--
Mavridis Philippe