Good question, Theebie. Here's some numbers.
In the description of a gigantic, ruddy-brown mekillot, it says they are 25 cords long, and twice the height of a half giant.
A half giant is about 150 inches tall.
The walls of Luirs Outpost are the height of two half giants (around 300 inches tall).
An inix is taller than a half giant (around 220 inches tall).
A silt flyer is taller than an inix (around 240 inches tall).
The walls of Allanak are 50 cords tall (750 inches).
A scrab is smaller than all of these things (98 inches tall).
Actually, to put it in real life terms:
Short distances are measured in inches (which bear a small resemblance to the English inch).
We'll have to assume that 'bear a small resemblance' means 'is equal to' for this to work. So keep in mind that these are inaccurate to an unknown, but not entirely useless, degree.
The average adult human height in the USA is 5'9"(five feet, nine inches), or 176cm for men, or 5'4"/163cm for women. The UK is a bit taller. (Men are a 1.5cm taller; women, .5cm)
Zalanthans are a little taller, the docs placing them between 5'8" and 6'6" (173-198cm)
A half-giant is between 10' and 12'6" (305cm-381cm). That's right, you stare them in the waist.
Dwaves are between 4'4" and 4'10" (132cm-147cm). Somewhere around wast-to-chest height.
Elven height range covers twice the area of human range. They are between 6'6" and 8' (191cm-245cm). Whenever they go to the roundear cities or outposts, they must feel like a tourist in Japan, excepting the politeness. The shortest adult elf is the same height as the tallest human.
Half-elves are between 5'10" and 6'10" (178cm-208cm), tending distinctly towards the human end of their parentage. It just goes to show that elves are a backwards, recessive peoples.
Information on gith is somewhat sparse, due to their habit of trying to kill scholarly explorers, but they are roughly 7' (213cm) tall, though it should be remembered that their natural posture is extremely hunched.
Halflings are the smallest of the widely known sentients on Zalanthas, coming in between 3' and 3'8" (91cm-112cm). It might be more accurate to call them "kneebiters", except that they like to start from up from the bottom.
The problems with measuring gith are further complicated with the mantis. The scant data which has survived such encounters puts them around 7' (216cm) tall. Interestingly enough, they are over half again as long! (as long as a half-giant is tall)
Quick note: For all sentients, the proportion of their height variance to their total median height is roughly the same. Very roughly.
Luir's walls are 25', or 7.6m tall.
An inix comes in at about 18'4" (or 5.6m) tall. Damn, that's a big lizard. A half-giant (or elf, if you can find one) on inix-back can see -over- the walls of Luir's Outpost, even when you assume riding low from the weight. (Eukelade, are you sure you didn't mean to say long? Just checking)
A silt flyer is taller than that, at a straight 20' (6.1m) in height.
A mekillot, finally, is one big mother of a lizard. As tall as Luir's walls (25'/7.6m, for those too lazy to look up), and a whopping 32' (9.5m) long. Probably easier to think of in yards - around 10 of those, or one tenth the length of an American football field. Just barely small enough to still consider lone travelers on foot as food.
Allanak's walls are 64' (19m), or about 21 yards, tall. An imposing scene in the desert, especially when you consider Tek's Tower is about twice as tall as -that-..
(As a side note, the ratio of a Zalanthan mile to an Imperial Mile is 1.25/1)