Your first question is answered in the EULA and TOU which you ahev already agreed to when signing up. To reread it go to settings (the cog wheel in the top bar) and scroll down and click on show licence.
IV OWNERSHIP
(...) YOU FURTHER ACKNOWLEDGE AND AGREE THAT THE CREDIT SYSTEM HAVE NO MONETARY VALUE AND CANNOT BE REDEEMED FOR CASH. NO REFUNDS WILL BE MADE FOR THE PURCHASE OF CREDITS. (...)
2) To adjust the size of the models go to settings and look for the setting called "Small size" and move the slide bar to your prefered setting, maximum is 60%.
3 and 4) Yes a little bit.