Well I am not sure if I really know what I am talking about, and this may
be stupid, but:
1) you can't change the size? Just delete the numbers it gives you and
enter '###MB' or whatever size you want to make the partition.
2) you generally want to have a smaller (50-100MB) root partition and a
larger /usr partition as a bare minimum, and if you have the space to
create individual partitions for /var and /home , you should do it. With
6.4 GB you have enough space. I am not sure about the intelligence of
using a 1000MB root partition only.
Just my 50 pesos, correct or not...