Spaced-out numbers

Python, 32 bytes

lambda l:'%%%ds'%len(l)*len(l)%l

An anonymous function that takes a tuple as input. Either numbers or strings work.

Example:

l=(1,33,333,7777)

'%%%ds'
## A "second-order" format string

'%%%ds'%len(l)           -> '%4s'
## Inserts the length as a number in place of '%d'
## The escaped '%%' becomes '%', ready to take a new format argument
## The result is a format string to pad with that many spaces on the left

'%%%ds'%len(l)*len(l)    -> '%4s%4s%4s%4s'
## Concatenates a copy per element

'%%%ds'%len(l)*len(l)%l  -> '   1  33 3337777'
## Inserts all the tuple elements into the format string 
## So, each one is padded with spaces

05AB1E, 3 bytes

Code:

Dgj

Explanation:

D    # Duplicate the input array
 g   # Get the length 
  j  # Left-pad with spaces to the length of the array

Try it online! or Verify all test cases.

JavaScript (ES7), 37 bytes

a=>a.map(v=>v.padStart(a.length,' '))

Input: Array of strings
Output: Array of strings

f=
  a=>a.map(v=>v.padStart(a.length,' '))
;
console.log(f(['0']))
console.log(f(['1']))
console.log(f(['2','3']))
console.log(f(['2','10']))
console.log(f(['4','5','6']))
console.log(f(['17','19','20']))
console.log(f(['7','8','9','10']))
console.log(f(['100','200','300','0']))
console.log(f(['1000','400','30','7']))
console.log(f(['1','33','333','7777']))
console.log(f(['0','0','0','0','0','0']))