File size is related to the resolution of the source video and the type of compression used.
Picture quality is related to the resolution of the source material and the type of compression used along with viewing distance to the screen.
Assuming the video source material is 1080p and compressed using is h.264, a decrypted BD rip will weigh in at around 20GB +/- some GB.
Such 'quality' rips can be recompressed down to as small as 1GB or less.
At an optimal sitting distance to the screen, viewing the 20GB rip on a 50" panel will look great while viewing the 1GB rip may also look good (depending on the video sceens).
With viewing the same 20GB file on a 110" screen at optimal distance, it will still look great but you would be sitting much further back.
The temptation would be to sit closer and when doing that viewing the 1GB file on 110" screen you may start to notice way too many compression artifacts on this screen.
Generally speaking, the more compression that is applied, the more noticable artifacts can become (and smoke or and dark sceens can suffer with more than other sceens).
That's where it all gets rather complicated as compression techniques can uses complex rules to squash different sceens at differing rates.
And some compression techniques are said to be better than others for a given file size.
Furthermore, the end result also depends on how the compression is applied (what/how you set the miriad parameters for the codec).
(In part that's why i don't reencode my BD rips along with the fact that HDD are not that expensive.)
Compounding all of this is the fact that some BD films are rather poorly mastered and some are filled with horrid artifacts (though i'm not sure if it's compression or decryption/unscrambling related).
Some BD's are simply poorly done upscaled versions of DVD's.
Some BD's are brilliant.
See
BD picture quality rankings for a look at the mess that is PQ on BD.
So the quality of the source material itself is variable (crap in crap out is a phrase that comes to mind).
Then if one talks about projection, not all projectors are the same (just like not all flat panel TV's are the same).
Some produce brilliant picture quality while others produce good quality for the same input material, some are just bad.
It all depends on the hardware within the panel or projector (and assuming you feed it great 1080p quality signal).
But it is still true to say that what you notice depends on how far you sit from the screen and in part how good your eyesight is.
Looking at the viewing charts within a
viewing distance article and/or
the optimum viewing distance wiki is just a start...
When looking to buy, trusting your eyes when viewing equipment in a similar environment to your home is even more important and the only way to go.
Asking how some unknown big file looks on a unspecified big screen projected by unspecified equipment is too much like asking how long a piece of string is
Looking at the RPi in isolation is for similar reasons not they way to consider it..
You need to consider where you get your source material from, how it's encoded and what you are viewing it on (video path and all assocuiated processing).
Me. i rip my own BD's and don't recompress and i don't stream (i don't have the bandwidth and i am data cap limited).
I use a RPi which outputs 1080p and has rather good hardware which copes flawlessly with BD and older DVD rips (note that i have VC1 and MPEG2 licenses installed).
I don't watch live broadcast TV with my RPi but one day i'll set it up again.
I have my equipment connected as RPi3 -> Pioneer AMP -> Pioneer 50" Panel.
It all works well though 50" TV has become very small by todays standard.
About the only downside is that RPi3 can't passthough DTS HD MA and other lossless formats like DD HD (so my RPi converts to 5.1 PCM).
Oh, and it sometimes looses connection to my blutooth remote (so i need to force a resync).
Picture quality wise, i'm more than happy with my setup (old eyes and all)
The search for a rollup screen and quality projector should begin at some stage (i'm not in a hurry)
And with spending $$ on a quality projector, i may need to relocate the RPi and use something better (or not) with it, guess i'll find out when i buy it.
But even if i had a magic box with perfect PQ, buying a crap projector will not result in a even good PQ dispite what the magic box sends.
It simply doesn't work that way