How much atmoshphere is there compared to land and water
71% of Earth's surface is covered with water and 29% land.  

Thinking in that regard, that's saying that on 29% of Earth's "surface" locations you have land below your feet, and in 71% of the locations, you have water.  So to continue in such terms, you'd then ask... ok, what percentage of Earth's surface locations would have air above them!?!  Well that's all of them.  So to if you're comparing it with those percentages, I guess you'd have to say it's another 100%.  Or, if we put them together into a full 3-dimensional surroundings at the surface, well it'd basically be 50% air, 36% water, 14% land.

But to compare how much of each there REALLY is, you need to include depth, getting some sort of 3 dimensional understanding of it.  But the picture that reveals is certainly not the picture we are used to from daily experience.  From the values I was able to find:



In terms of the room each takes up, the volume:


Surface water (oceans+lakes+rivers, glaciers, etc) is 1.4 billion km³
The inside of the Earth is about 1 trillion = 1000 billion km³ 
For the atmosphere, as mentioned in comments, it's a little more difficult, as the gases only gradually give way to space.  You find less and less gas as you go up, but there's no set spot where there is none, as some tiny amount is always floating off into space.  So where do you draw the line?  A commonly used boundary between the atmosphere and space is the Karman line.  This would lead to a volume of 53 billion km³ (using Earth's radius = 6371 km).  Note that only about half of that is in the troposphere and stratosphere, which are perhaps the familiar zones of the atmosphere where weather and the ozone layer (and 95% of air molecules) reside.


To compare these directly, the volume of land is 20+ times larger than the atmosphere (or 40+ times larger than the troposphere+stratosphere), and 750+ times larger than the Earth's water.

Or as percentages, the volume is:


Land 95.3%
Air 5.6%,
Ocean 0.1%




Another useful way to look at is mass.  In terms of how much mass there is of each (from NASA):


Atmosphere: $5.1\times10^{18}$ kg
Hydrosphere: $1.4\times10^{21}$ kg
Geosphere: $6.0\times10^{24}$ kg


So there is 500+ times more water than air... and 4000+ times more "land" than water.

Or in percents, the mass is:


Land: 99.98%
Water: 0.02%
Air: 0.00008%


You could also look at the comparison in other ways, such as with the number of molecules.  But to save time, the answers will still typically boil down to the same idea: though land is the least of what we see around us on the surface (if you're a true global traveler), it is by far and away the greatest part of our planet overall.

One final visualization that looks interesting is considering if we made giants balls of the air and water at Earth's surface... they'd actually be about the same size...  and nowhere close to the size of Earth (and that's even neglecting the fact that for fair comparison, the Earth, if somehow all of it could be brought to surface conditions, would be even larger due its current internal compression).



As to whether the amount of oxygen is changing, remember no element can be created or destroyed (aside from nuclear processes).  So the only way oxygen can change is if it is tied up in a compound or is removed into the ground or space.  In our lifetimes, there is very insignificant change overall, though it sounds like a notable amount is being used up each day in combustion reactions (see the part about oxygen depletion further down) [oxygen decreases from that, but since carbon dioxide has more mass, the overall atmospheric mass actually increases, just not the stuff you can breathe!].  If you're looking for a much longer history, it appears this Britannica article gives a more thorough discussion of the factors in play and some of the changes that have occurred.

As to the mass of the atmosphere over eons, I naively thought maybe less would be known about that... but articles in Scientific American and Nature both hint the atmosphere was at much different pressures once upon a time (albeit reaching conclusions in opposite directions!)  Such changes in pressure would likely come with large differences in mass and/or volume of the atmosphere as well, but until someone shows me otherwise, those estimates would seem to be quite speculative.

And if we're going that direction, maybe we should just turn to What-If XKCD!