I put a "a cup of tea" and it was marked wrong. But nobody in english asks for a glass of tea. Because tea isn't served in glasses, but cups.
Hmm, I have a feeling that there will be various answers to your question since it would depend on the dialect. My family and I are southerners, so to me "a teacup" = "một tách (nhỏ)" and "a cup of tea" = "một tách trà". We do specify that "a teacup" or "a cup" in general is "một tách nhỏ (small)" and "a mug" is "một tách lớn (big)". Keep in mind that's just how my family and I say it.