In SSMS if you connect to the SSAS instance and right-click on server and go to 'Properties' in here are a number of Memory metrics.

The initial one to look at is:

TotalMemoryLimit

This will specify the maximum amount of physical memory the SSAS instance can use

Below a value of 100 this is expressed as a percentage of the total server memory and above is expressed in bytes.

http://msdn.microsoft.com/en-us/library/ms174514(v=sql.100).aspx