I would like to know if I can increase the font size of the questionnaire.
Please see Survey Layouts for detailed instructions.
I want that my participants can be assigned randomly to one language on the first page rather than choose one at will.
Sure, you do not confuse them by doing so? Reading a German invitation an then starting with an English questionnaire (or the other way around) may look like an error. You may have an introduction in the invitation language and then (depending on experimental group) explain what is ahead of them. You will also have to ensre that they speak the other language, or you will have a systematic dropout, confounding your experiment.
The randomization itseld then just requires a random generator and the function setLanguage() used in a IF
filter.
And, of course, you'll have to set a default language in compose questionnaire -> settings.