You can use the vSphere client to connect to your host?
ESXi itself has a very low footprint in terms of memory. In production enviroments I see the hypervisor itself use somewhere around 200-300 MB. You could do 2x 6GB for your VM's and have 3GB still "spare" for another small VM in the feature. If you want to allocate memory the best I'd go with configuring Windows at 8 GB and Ubuntu at 6 GB, or just the other way around depending on the functions they perform.