On my world sulphur dioxide makes up most of the atmosphere (42%). It comes from constantly active land-based and Hydrothermal vents. I know there are basic, or basic compared to a human, organisms that can 'breathe' sulphur dioxide. Would it be possible for a sentient and relatively large creature to breathe sulphur dioxide?

So my questions are:

• How efficient would my organism have to be to breathe it a rate they can live by?
• How large would their lungs have to be or would they have to have some other system of breathing?
• How large could they grow at a maximum? Or would they have to remain small?
• What are the downsides of breathing sulphur dioxide over oxygen?
• Are there any upsides?
• Would it even be possible for them to breathe it at all? Even if they evolved in the environment would it just not be possible to sustain life there?

*And as a secondary question if you wish to answer:

• What other effects would it have on the ecosystem and world?*

(Sorry if this has been asked before, I searched this site and other sites but I found little that could help. Thank you in advance.)