Hi Mehrdad That's something you could say in front of an audience, but less so when you talk to someone personally. If it's directed towards one person only, "thank you very much" will sound more natural and spontaneous. You could replace it with other common expressions, such as: - thanks a bunch - thanks a million - I owe you - Cheers! - Thanks so much. Hope this helps.

To me, 'A big thanks' sounds odd. A big thank you, or Big thanks would be OK. As J.Arthur said though, it sounds like something to say in a speech, rather than in a one to one situation.

Agree with J. Arthur. If you’re in front of an audience and want to make a grand gesture, you could say, “A big thanks..” but otherwise it’s not really used.