Reporting Services can be demanding in terms of memory and CPU, so it often makes sense to separate it from the database server element of SQL Server - obviously this isn't cheap in terms of licencing. One thing to bear in mind is that the metadata for SSRS is stored in a SQL database, but this can be on a different box. Where SSRS gets the business data from depends on where that data resides - it could be anywhere, but it might be a SQL database somewhere.
If your SSRS deployment uses subscriptions to generate reports out-of-hours, and your SQL database is used for other requirements during business hours, then it might make sense to keep them on one box.
Depending on your SQL edition, and your requirements, you can also treat the separate SSRS boxes like webservers, and have them in a high availability Network Load Balanced cluster - you can't cluster SSRS in the same way that you can with the database engine for high availability.
Like anything, try to test it all in a pre-production or Proof Of Concept environment.
Intuitively, if I was doing an OLAP solution for a retail chain, I'd say your infrastructure is really inappropriate for a system with substantial data volumes. This sort of kit has trouble with the data volumes you get in insurance, which is probably a couple of orders of magnitude smaller than I would expect to see in retail.
As gbn states in the comments, SSRS is just a web application. You can set up a farm - start with one server with a few GB of RAM and a couple of virtual CPUs. Monitor it, and expand it if it's overloaded.
The amount of disk space used by SSAS depends on the volume and the aggregations you put on the cubes. The storage format is quite compact - more so than SQL Server, but if you have large volumes of data it will start to get quite big. If it's getting into the 100+GB range then you should also look at partitioning.
A surprisingly applicable generic solution
Now, your client probably doesn't want to hear this, but VMs are not my recommended hardware configuration for any business intelligence solution. They work OK for transactional applications or anything that is not too computationally intensive. BI for a retail chain is probably a bit aggressive for this sort of infrastrucutre.
As a generic 'starter for 10', My recommended configuration is a bare metal 2-4 socket server like a HP DL380 or DL580, and direct attach SAS storage. The principal reason for this is that a machine of this sort is by far the best bang for buck as a B.I. platform and has a relatively modest entry price. If you put a HBA on the machine then you can mount a LUN off the SAN for backups.
IOPS for IOPS, this sort of kit is an order of magnitude cheaper than any SAN-based virutal solution, particularly on sequential workloads like B.I. The entry level for a setup of this configuration is peanuts - maybe £10-20,000 - and it's a lot cheaper and easier to get performance out of something like this than a VM based solution. For a first approximation, the only situation where this kit is inappropriate for B.I. work is when the data volumes get too large for it, and you need something like Teradata or Netezza.
What can you do with your VMs, though?
A good start would be more RAM - try 32GB. SSAS is a biblical memory hog, and you've got slow infrastructure. If you're stuck with a VM, put as much RAM on it as possible. Disk access is slow on VMs.
A second avenue is to partition the cube over multiple LUNs, where the LUNs are on separate physical arrays. Not all SANs will give you this much control, though. 80GB is getting into the range where you might get a benefit from parititioning, particularly if you've got a platform with slow I/O.
Tune the buggery out of your cube aggregations - try usage based optimisation. The more hits you can get from aggregations the more efficient the server will be.
Without measuring your workload, I doubt anyone here is in a position to make recommendations that are any more specific than that. Although generic, the pointers above should be a reasonable start if you haven't implemented them yet.
Best Answer
When you install SSRS 2012 in SharePoint Integrated Mode it does not install an instance of SSRS on the server like it would if you installed it normally. So you have to configure it via the SharePoint interface. This is the reason you did not see Reporting Services running on the server.