Eurostar luggage allowance
Eurostar describes what you can bring instead of publishing measurements. Their rule is shown below in their own words, and never turned into a yes or no.
What Eurostar says
Eurostar, in their own words
Each piece of luggage can be up to 85 cm long on routes to and from London, or 75 cm on all other routes. There is no weight limit on your luggage, but you must be able to carry your bags safely on and off the train and store them in the designated areas. Eurostar Standard and Plus include 2 pieces of luggage plus 1 hand luggage; Premier includes 3 plus 1. "Hand luggage" covers items such as backpacks, handbags, briefcases and laptop bags. Children may take one piece of luggage and one piece of hand luggage. Pushchairs and prams travel free if folded away. Extra bags, oversized items, bicycles, musical instruments and sports equipment are carried for a fee, and both the fees and the maximum dimensions differ between routes to and from London and routes between Belgium, France, the Netherlands and Germany.
Before you rely on this
An empty field on this page means the operator doesn't publish that figure. It never means there is no limit.
Meeting a published limit is not a promise that staff will accept your bag at the gate. At least one operator states that paying at the gate does not guarantee a bag over the limit will be carried.