The Kitchen Garden Series small linen tea bags are great for a single cup or small pot of tea. Flip them inside out to wash them in the sink - they'll last a lifetime!
Tea bags measure 2.5" x 3", and are available in natural white or natural ecru linen.
This teabag works great! Feels a bit big for a regular coffee mug, but it'll work just fine in a mug or in a teapot. We had to let it steep a little longer than normal, but otherwise didn't need to do anything different.