Here are a few opinions:
1. X models range from 12.5" to 14", while T models are available in 14" and 15.6" versions. It really depends on your work schedule and work environment: Are you going to use the ThinkPad on-the-go, or will you be working primarily from a well-lit office? Is Excel the only office program you'll be using - what about document-writing or visual presentations?
2. Personally, I would opt for a 14" screen for Excel chiefly because of the 16:9 ratio used in modern laptops. I have a ThinkPad T61 with a 14" screen but it's a 4:3 ratio and that gives me a more useable work area compared with my 15.6" IdeaPad Y50, especially for Excel-style data processing. The 16:9 ratio in the Y50 is great for entertainment and...