You should be fine on the standard seats. Eurostar is now old rolling stock. The trend for ridiculously cramped conditions hadn't really set in when those trains were built. I'm 5'10" (wide* as well as tall) and I'm OK in the standard seats.
*I always try to get an window seat on a second class British Rail train as otherwise I have a trolley shaped bruise on one or other side, just below the shoulder.
Not quite true. The service that goes to Amsterdam uses Siemens Velaro D trains (aka ICE 3 family), unlike the older TGV derived stock.
But the chairs are fine, I have a large arse and have no issue fitting in them.
If you're taking time around europe weigh up the cost of an interrail pass option, Eurostar costs 35 euro each way with one but unless it's changed you can only book the seat through SNCBs website (which is surprisingly decent)
With Interrail, AVE, SNCF and Thalys are a pain in the arse due to compulsory reservations, Germany is generally easy even ICE is on it without compulsory reservation (excluding some border crossings such as into Denmark) though some land level operators aren't on it, where as some S-Bahns are. Swizterland is a minefield of private operators but easy enough to work out. I could og on for ever better just reading Seat 61 if you haven't already.
Agreed with the interrail pass. They are really useful.
They recently brought out a new pass, 4 days in a 30 day period. For €246. Or about €65 a day. I used this to get from Amsterdam to Copenhagen, I didn't need to reserve, and it was cheaper than buying a direct ticket each way. You only need to reserve on the cross border DE<->DK train in the summer months. The downside is that using an interrail pass in France is a right utter pain in the arse, cos France... But if you avoid France and play with the rest of Europe, it's great. I used one a few years back to get all the way from Canterbury to Crete, and back to Utrecht, without needing to fly (we did go through France on the way out, which was a faff).
J