Hello Rolf,
I bought the player for a customer (after a rather mediocre experience with the Sony BDP-S6700) and the customer is now happy. The DP-UB424 has an excellent price-performance ratio! However, I did some research and found the following:
"You can change the position as well as the size of the subtitles via a zoom function during operation."
Unfortunately, in the operating instructions I only found how to select the "subtitle menu", but not the available setting options.
I am therefore basing my statement on information from the Internet, i.e. "without guarantee"!
I hope I could be of some help.
Have fun & LG, Andreas