You could try the Handcent SMS app from the market. It is very customisable... For the first part of the Menu, your Inbox as such you can change up the appearance of the - Menu>Settings>Custom Style>Conversation List Settings then subject font lets you increase the font size. You can also set the font size for your contacts on there too. Then for the actual conversation thread you do Menu>Settings>Custom Style>Bubble Settings you can set the fonts for your sent and received messages within each thread bubble. If you get the app what I have just typed should make sense! If you need more help with it, let me know.
