Step 1: Open your osu! folder and find your skin folder
Step 2: Go into Staiain skin folder and look for the skin.ini and open it (I'd recommend Notepad++ for it)
Step 3: Look for the Line with ColumnWidth: 30,30,30,30
(numbers are just an example, I don't know the actual numbers in Staiains skin)
Step 4: Change the numbers, the first number is the width of the first column, the second number the with of the second column etc.
Step 5: Save the skin.ini with CTRL+S.
Step 6: Reload skin in osu!, either by switching to another skin and back or using CTRL+ALT+SHIFT+S.
Experiment as you wish.
EDIT: just corrected a small grammar mistake