Winamp Double Size

Depending on the modern skin, you can click and drag on the lower right hand corner to enlarge skin. Otherwise, Ctrl+'Right Click' on a empty space and select Scaling. Scaling lets you change the overall size of Winamp, instead of just increasing the window's dimensions.
For classic skins, you can double size the Equalizer and/or Main window by using the Ctrl+D shourcut key. Other classic skin windows (playlist, video, ML, etc.) are resizable via click and drag on the lower right hand corner. You can have smooth or micro-fine sizing of resizable Windows by, Winamp > Ctrl+P > Skins > Classic Skins > 'Classic Skin Options' tab > checkmark 'Enable smooth window resizing for resizable skinned Windows'
The above tips may or may not apply to third party plug-in windows.
You can change the size of the playlist font/text..

1. Ctrl+P (for preferences)
2. Options > Display
3. change the number for 'Playlist font size'

Alternatively, use the Ctrl + or Ctrl - (plus, minus on keypad), while playlist is in focus, to incrementally change the text size.
To change ML text size

Ctrl+P (for Preferences) > Media Library > Library Options tab > checkmark 'Media Library uses same font as Playlist Editor'

The ML font will reflect the playlist editor font.
Note: On some SUI (single user interface) skins, like Bento, scaling the skin may have undesirable results