Unfortunately, it seems that in Tableau, your columns must be the same width - you can't change the width of individually right now.
(But it's an improvement you can vote on!
But unlike columns, your row values can all have different widths - both customizable and auto-sized based on content. So my question is why you set up your table the way you did, with multiple mark types? That's really only valuable if you're doing a lot of complex coloring and symbols within a text table. (I notice you have non-working Measures labeled "Color", so I'm assuming that's what you're going for here - a Jonathan Drummey-esque crosstab of many colors?) http://public.tableausoftware.com/views/conditionalformattingv4/Introduction
If you can just make the CLUSTER your row, rather than try to fit it into a text field like you're doing, you'll get the most flexibility. Only thing you're prevented from doing is coloring that text based on conditions. But you'll get the row widths you want, and will still be able to do conditional formatting on all numbers. And you can always add a symbol or bar next to the cluster name that matches the coloring you want.
In short, you have a few options, none of which are perfect, and I think unfortunately you'll have to decide which one is least problematic for what you're trying to accomplish.