Smallest font and thinnest line for shirt?

Hi. I'm clearly a newbie, and I hope this hasn't been posted before. I searched the forum and didn't find anything.

I'm wondering what are the smallest font and thinnest line that can be used in a tshirt design that will still be legible/show up? I'm thinking of small text for copyright notices and thin lines for...um...thin lines.

If it matters, I'll primarily be using Spreadshirt to produce my shirts, at least initially.

Thanks!