This is on a Mac 10.10 using Safari 8.0.
If you are typing a body of text and make a spelling mistake, if (1) it does not get automatically corrected (a misspelt word), and (2) you do not correct it yourself, then a red line will appear below that word.
Assuming you continue typing and correct it later using the computer's "suggestions" dialog, upon selecting a suggested word, the incorrect word will be replaced, but a portion of the text that surrounds that word will reduce in size with [SIZE=2][/SIZE] tags. This only happens on occasion, and I cannot determine in what conditions it occurs.
However, sometimes the text will resize when the misspelt word is automatically corrected by the computer. So, both what I posted above (manually selecting the correction from the list), and when the word is corrected "in-line" may cause this problem to occur.
I will say that this problem occurred when composing this message (I'm not sure if that's relevant.)
For reference, these are my settings in my "Spelling and Grammar" menu:
This is how the message looks in the post editor when this problem occurs. Note: the word "samples" is the word that was corrected prior to it having a red line below it.
Here is the same message in the "BB Code Editor":
I will say that, although I have tried to deliberately produce the error, it does affect me under normal circumstances.