When using DES, how large would the key space be if we only use letters as characters?
A DES key is 56 bits. If we assume letters are ASCII encoded and take 8 bits each this means a DES key of only letters would be 7 letters long. Given that there are 26 letters in the roman alphabet we would have $26^7 \approx 2^{33}$ possible keys, or if we allow uppercase and lowercase, $52^7 \approx 2^{40}$ possible keys. Both keyspaces can quickly be enumerated with modern hardware and can easily be brute forced.