Here is a guide to change the resolution of the grub loader and the plymouth theme. If you have a big and blurry grub loader/plymouth theme you can use the startup manager to change the resolution. Be sure to set the same resolution for the plymouth theme as you use for your grub loader.
First you need to istall the startup manager, open a terminal window (press the SuperKey and type: terminal) and copy+paste the following line:
sudo apt-get install startupmanager
To start the startup manager press the SuperKey and type: startup manager.
In the Boot options tab you can change the following: timeout settings, the default operating system, display resolution and color depth, show boot splash and show text during boot.
To change the resolution click on Resolution options and select the resolution you want. And if wanted change the color depth to 24bits.
The Advanced options tab allows you to make changes to your bootloader menu resolution, click on the resolution options and select the same resolution you used for the Display resolution (Boot options tab). When done click on the Close button and when the startup-Manager is done and has saved the new settings you need to restart your computer and enjoy your grub loader and/or plymouth