What size luggage will be best for a 5 week trip?

I'm studying for 5 weeks in the UK and I have a 28" upright already, but my professor said to travel light. She also keeps stressing the fact that we should bring a piece of medium sized luggage since we will have to take the Tube, the Heathrow Connect or Express when we arrive in London to get to our final destination. What sizes are considered medium? I'm pretty sure my 28" is out of that category. I don't want to lug that thing around on the Tube or on a train. I'll need something that can accommodate my clothes, a pair or two of shoes, and other travel essentials (i.e. toiletries). I also may do a little shopping while I'm away. Would a piece of luggage that is 25" or 26" be sufficient? I've bought compression bags for my clothes, so they should not take up too much space.