Can block ciphers encrypt messages larger than the key length?
A block cipher like AES itself cannot be used to encrypt large files. One needs to use some mode of operation to chain a sequence of blocks. The length of this whole "chain", which essentially is the product of the block size and number of blocks, is the length that you can encrypt files with. The key size of the block cipher basically tells you what security level you can get from a "chain" of block ciphers (or a single block cipher of course).