Is it ethical to accept small gifts from students? And how small is small?
In the general case, professors/lecturers/TAs should not accept gifts from students, as these things can lend themselves very easily to accusations of conflict of interest. This falls in the same general category as dating one of your students, or grading your kid's exam.

That said, I can think of three situation where gifts are acceptable.

(1) a graduating student gets you something nice after graduation (e.g., in the department where I did my PhD, it is normal for students and advisors to exchange gifts after a successful doctoral defense).

(2) a current student makes a "collective gift", e.g., a student brings a cake to class to celebrate his/her birthday and offers you a slice.

(3) you go through some major life event and students pitch in to get you something, e.g., when I came back to work after having to get major surgery, the grad students in my department got me a nice coffee machine to celebrate that I hadn't died.