Not sure about the Etrex, but I thought that generation of Garmin handhelds you could reconfigure the view (map or navigation screen?) to show 4, 2 or 1 data item. (My GPS-V form 1998ish does that , and it is way pre Etrex). Needless to say if you set it to "one data box" the font size in nice & big. I thought you could i nthe "data" screen o nthe etrex tel lit what you wanted to see and what font size you wanted it in?
Alternatively, do you use the etrex for any navigational stuff? Assuming it's not one of the "MK1" Etrexes, you could get a GPS speedo that reads NMEA 0183 & use the etrex to "feed" it speed data.
I'm sure I've seen what you describe on my internet meanderings, just not sure what price as I wasn't really paying them much attention having configured my Garmin to show speed in large friendly letters.