Now I used my graphic program to measure how high and wide the graphic available on TFP website is and using that graphic, I would guess the large type (first sentence) is approximately 10-14 inches tall. The smaller "disclaimer" text is approximately 6 to 8 inches tall. I have a picture of a DART bus (credit being given here to Dallas Area Rapid Transit with a quickly pasted, resized copy of the ad on the side. It is quite readable (all lines of the text). I can also note that the opposition had no problem reading the name of the org responsible for these ads as well as getting the phone number to TFP in order to spread mass hysteria, and that text is exactly the same size as the "disclaimer" text. The only difference is that text is bolded while the "disclaimer" text is not. It can also be noted that TFP has a slogan (which is on this ad as well) and that slogan is half the size of the disclaimer text and again, the opposition had no problem reading the slogan either.
(Click picture for larger version)