Hi,
I tried the issue on my computer it worked fine. Once you start writing on the Math Input Panel and reach to the right corner the panel will automatically expand. If this does not happens in your tablet then I suggest you to go to Turn Windows features off and on and turn it off and then turn on again and check.
Follow the steps given under the section “**How do I turn features on or off?”**of the link given below. Follow the steps given to turn off the feature and then you can follow the same steps to turn in ON. From the list of Windows Features chose Tablet PC Components. Check the picture given below.
Add features: FAQ
http://windows.microsoft.com/en-CA/windows-8/add-features-frequently-asked-questions
Let us know the status.