What does 'telinha' or 'na telinha' mean?
I can't find an English definition in the dictionary.

Question

Answer

You won't find it, it is oral language, often used by TV channels. See this explanation:

Letteraly:
TELA = SCREEN
TELINHA = Small screen

TV channels have been using this expression in a informal try to expose their schedule programs. Nowadays it is definitely well known all over the country as a synonym of TV set. 'Telinha' means TV in opposition a 'TELONA', the big screen (cinema screen), but TELONA is not often used.

Answer

''-inha'' indicates a diminutive. "na telinha" can signify "on the small screen".. or "on the TV" ..a popular language.