I need to increase the width of some of my cells within a matrix im using.

You must be referring to the Matrix Table field on your jotform  http://www.jotform.us/form/41756505487160

You can adjust the cell width and table width within the Properties of the Matrix field.

Hope this help. Let us know if you need further assistance.

ok thanks - but what metrics is used? If I put in 100 in table width is that 100 cm or what does it mean?

I tried to mess around with it but it doesn't seem to work for me. Each time I change the table/cell width it results in a small matrix than I previously had. 

I basically want the table and cells as wide as possible.

Could you let me know how I achieve that?

Metrics that are used are pixels (px). You can also increase the width of entire matrix table.  

Here is my demo form where I have set Table width to 1000xp and Cell width to 150px.

http://form.jotformpro.com/form/52565306614959? 

Hope this will help. Let us know if you need further assistance.