Is AES easier to crack when the input is small? Say you just want to encrypt a number. For example, say the number could be any  double. A double in C# and Java is 8 bytes.

If you were to encrypt a double using AES (MS-Doc, defaults to CBC as the mode):

var cypherText = AES.Encrypt(123d); // 8 bytes


would that be trivial to crack? If not would it at least be significantly easier to crack than the cyphertext from a larger input:

var largeText = GetDeclarationOfIndependence(); // 6760 ascii characters, so 6760 bytes
var cypherText = AES.Encrypt(largeText);
Assuming that encryption is performed correctly:


Using a modern cipher (AES)
An appropriate mode of operation 
With no bugs in the implementation


then the nature of the plaintext has no effect on the security of the ciphertext. 


  If not would it at least be significantly easier to crack than the cyphertext from a larger input:


Actually, a longer ciphertext is technically easier to "crack" than a small one. 

The reason being that the larger cryptogram provides the adversary with more known plaintext-ciphertext pairs. Attacks that do exist against modern ciphers typically require very large numbers of plaintext-ciphertext pairs.

That being said, this effect is still negligible in pretty much any realistic use case. The quantity of plaintext-ciphertext pairs required for cryptanalysis is frequently obscenely and prohibitively large.