So, select the subtitle in the design window and use the text toolbar to change your font size. What problem are you having? (Hint: This is covered very well in the Help file and the manual.)
Then that's what you should've asked for. ;-) At any rate, as far as I know, you can't change the default font settings for subtitles.
What you can do is change one subtitle and then apply its settings to the others in your project. Change one subtitle to the setings you want. Then right-click its textbox on the timeline and choose "Apply Formatting to Selected Tracks." Any new subtitles you add will use the original settings, so either a) follow this step again after you're done adding subtitles, or b) copy and paste subtitles events that you've formated instead of inserting new ones.
Oh those subtitles...I still remember. The begining is difficult as always, but as soon as you realize as everything works, making subtitles with DVDA is a piece of cake. May I suggest you do something? Choose Arial, Bold and 22 for the size. I have tried myself many other sizes and font types and these are the best settings....believe me ;)