Even after adjusting the margins, I found that TMDSAS, unlike AMCAS which has a 5300 limit, has a 62 lines limit with 80 characters per line. Either way, the character count limit is not the same. I found myself editing and cutting out sentences to ensure everything got in. Once you click save at the bottom, you can see if everything was saved. If you want to double check everything, ctr+A everything in the text field and paste in Word do a word count. If the word count is the same as the original, nothing was cut. This should be full-proof.
80 times 62 = 4960. Your AMCAS PS will just have to be adjusted regardless, if over this limit. If you are applying to both systems, it may help to write a PS that has this limit, instead of 5300.
Best regards.