Open 'Lesson 3 - Implementing and Tracking Options (LBaP).gbsproj' Click on the little purple square over the PRESS START sprite. Look at the right-hand menu to see it displayed as sprite from a sprite sheet. The PRESS START is a sprite from the sprite assets folder. You will have to create a new larger sprite, outside of GB Studio, for PRESS START and then replace it.
Click the top-left-drop-down menu and select Sprites. The sprite is saved as press_start.png
Peter Milko sprite tutorial: