Synodontis Eupteras

[coyemuse]

what size tank would I need if I want just my syno in it?

 

[the_lock_man]

110 litres, something like 4'x1'x1'

 

[N0body Of The Goat]

110 litres, something like 4'x1'x1'

I'd agree on the length as a minimum, but something a little wider and deeper would be better (15" minimum) IMO, to help the fish turn around as it gets older and bigger. These are not small fish as adults (planetcatfish states 222mm SL) and can easily live beyond 20 years like many synos, if given good care.
 
Going on your sig (corymuse), I'd say it would probably be easier to keep the syno in the 75g and find a smaller tank for any current tankmates that you have concerns about.