I have the same problem. I use a magnifying glass sometimes to try and see the very small red line. I would prefer the word have a heavy line under it, or highlight the entire word red.
I hvae been a user of WP since the 1980's and love your program. It would be very helpful if you could allow the red color for underlining misspelled and unrecognized words to be adjusted to darker as my eyesight as one in my 70's is not as sharp as it was in the past.