Even if you went for a minimum size 3 inch beam (width and height) you would be bringing your ceiling height down low. 8ft is an average European home ceiling (2.4m)
Adding beams would emphasise a low ceiling and (I find beam removal is quite common for European houses with similarly low ceilings); So I would be cautious about recommending their addition to your ceiling.

Yeah, that was what I was thinking too. Too bad. I guess I'll just admire them on houzz!

First, you must determine where the rafters sit on an outside wall. Some extend outward with front porches. Then they will be flush with an exterior wall. This is a major problem. With a wall that is not the load bearing wall, you could wind up with ceiling pitches that are not the same. Go into the attic and determine where the roof joists really go. If opposite sides of a room are roof bearing, you can wall-board over the existing roof rafters. Be prepared to insulate this area between the roof and the rafters. If it is not stick built (no trusses) It is way to costly. Also, the wall board must be on rafters that are at least 2x8, or the ceiling will have condensation marks because of the temp difference of the wood and the insulation. Most vaulted ceilings start with 2x10 rafters.

Reads like Henry is recommending raising the ceiling. We've had numerous discussions on Houzz about how to do that properly. It is a building science issue and I recommend research...at the very least reading my posts on hot/cold roof technology and the issues surrounding each. ASV is correct about the cramped feeling of adding beams to an 8' ceiling. Codes in the US allow an average ceiling height of 7' with a minimum of 6'8 at doors and trims...Just so you know!