Can't be bothered to find the data but purely for discussion purposes as there was only one other reply:
"How much are high-bypass turbofans bigger that 1950-s, 1960-s pure turbojets or low-bypass turbofans, comparing engines of similar thrust?
Also, are they heavier or lighter?"
Reduction in weight is important for aero engines. The heavier the aircraft, the greater lift required, requiring greater airspeed, thus greater thrust and therefore more fuel is consumed. The heavier the engines, the more fuel used to get from A to B. Therefore you'll find engine manufacturers will only increase the weight of their shiny new engine design in comparison to an old one of similar thrust if there's a much more compensatory increase in efficiency as a result. Bluntly speaking that's never going to happen. So what I'm trying to say is that RR, IAE etc. will work to reduce the engine weight in design and so you SHOULD find them to be LIGHTER.
"I have heard quite some planes have actually had engines removed and different engines installed"
Yes - for example, check out pictures on a*rliners.net of BA's 747's in the 80's and then do a reg search for the same aircraft today if still flying.
"And pure turbojets have more noise than high-bypass turbofans,"
This is due to the pressure fluctuations of a single hot jet stream. In a turbofan, the hot streat is surrounded by a 'cold blanket' from the bypass flow. This reduces pressure fluctuations which we hear as sound, i.e. noise.
".. as well as having more fuel burn per thrust."
Again, yes but turbofans can't reach supersonic speeds, and in these situations a turbojet must be used regardless of the lower efficiency.
"Does it mean that a high-bypass turbofan will not work well near another turbofan, as they compete for air to suck in?"
There is plenty of air to go around
And if there was a slight, very slight decrease in the local air pressure at the inlet nozzle entry, it would have no calculable effects on the efficiency of the engine (which takes into account the air pressure on entry to the compressor).
"Could high-bypass turbofans be mounted near each other?"
Yes, but there are no additional benefits to doing so. However, maintenance practices would be restricted, and in addition high-bypass turbofans such as the RR-Trent or GEnx are way too big / noisy for mounting on teh side of the fuselage, or stuck to eachother... etc.
