I'm not sure why your code did't work. I did something very similar and it worked fine on both Firefox and Chrome.

I was successful with the following:

#q7 select {font-size: 8px}

where q7 was the id of the table. This did not work using the id of the column (which in Laserfiche Forms is implemented as a <td> element.

The following also worked but of course is much less specific:

select {font-size: 8px}

I don' t have your code to experiment with but I suggest you try separating out the setting of the font size on the drop-down on its own line of CSS similarly to one of the above examples. As you learn more, consider posting your results here.