Dictionary size
Specifies Dictionary size for compression method.
Usually, a higher Dictionary size gives a higher compression ratio. But compressing can be slower and it can require more memory.
Memory (RAM) usage for LZMA compressing is about 11 times more than dictionary size. Memory usage for LZMA decompressing is close to value of dictionary size. Memory usage for PPMd compressing and decompressing is almost equal to dictionary size.
Word size
Specifies the length of words, which will be used to find identical sequences of bytes for compression.
Usually for LZMA and Deflate, big Word size gives a little bit better compression ratio and slower compression process. A big Word size parameter can significantly increase compression ratio for files which contain long identical sequences of bytes. For PPMd, the Word size strongly affects both compression ratio and compression/decompression speed.