I find 1 regular key height to be fine for all keys, I don't prefer tall keys. Really, I guess a VBN centered 3 key wide space bar would do it for me and allow me to hit the alts with my thumbs. I can hit an XC Alt, but not a <> alt with my thumbs. Thumb keys require that 1.5 key width, where as the others require regular key width.
It is more important that there is only one key between the end of home row and the enter, than how wide enter is. Although, I certainly would not want something on the other side of enter.