I don't think there are any solid rules governing the "size" words. Garnered from observed usage, here's the order that *I* think of them (and use them) in, from smallest to largest.

malgrandega
eta
malgranda
granda
ega
grandega

malega and maleta are questionable (to me), so they don't make my list. They sound nice, but we already have words that (again, in my opinion) adequately cover that ground (especially if you equate eta to malega, and ega to maleta). So I generally avoid their use, but don't outright ban them from my lips/fingers; they are grammatical, and thus technically allowable.

Though also grammatical, I advise against the use of etega and egeta (and their mal- variants[1] ), entirely. It isn't clear what either word really means - whatever case you make for one meaning, it's just as easy to make the case for the opposite. You're playing with dark forces with these, and I can not be held responsible for the results of their usage (probably mainly just confusion, actually, and possibly an annoyed smirk of repressed derision for using such silly words).

Any other combinations (malgrandegeta, and the like) should only be used if you are trying to be funny and/or a smartass. The pieces aren't that solidly defined in the first place, so compounding them in large numbers doesn't exactly help do anything except make the resulting words foggier. It's like saying "You have to go south by southwest by northwest by northwest by southwest. Got it?" "Um... what?" Precise, yes. Comprehensible, not so much.

As a matter of style, because they are short, I tend to prefer ega and eta for big and small. But I will happily resort to the other words if precision, context and/or euphony demand it.

As for what exactly each one means: ha. Top to bottom, I give them ordered smallest to largest. eta is smaller than malgranda. grandega is bigger than ega. And so on. Quantum particles are malgrandega. The universe is grandega. Baby kittens are eta. Or maybe they're malgranda; does it really matter? But compared to a quantum particle, a baby kitten is grandega. Like that. It's all relative. You want to get more precise, start using measurements. "127.3 lbs" nails it[2].

Also, and again, just to be clear, that's MY list, and these are just my opinions. These aren't statements of official rules, and other people might think different (you will notice that my list is slightly asymmetric, due to the use of the suffix -eg as an intensifier rather than an "enlarger"[3]; -eg thus appears at both ends of the list). But that's how I tend to use the words. I may not even do it entirely consistently, and thus probably will not be arguing with anyone about how on-the-money my list is.

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[1] Especially their mal- variants. You do want your listeners to understand you, right?
[2] Obviously not referring to the kitten.
[3] By which logic, ega should mean "more intense" rather than "big". But it doesn't.

So if -eg is an intensifier and -eg is the opposite, then something like varmeta would be perhaps lukewarm and something like alteta would be in English something like "tall-ish", while malalteta would be shortish.

Again, if I understand you right, malalta would be short. malaltega makes that *very* short whereas malalteta makes it still somewhat short, but not as short as the basic malalta, leaving the diminishing order:

height:

Altega: Very tall
Alta: tall
Alteta: tall-ish, somewhat tall
normala: normal
Malalteta: short-ish, somewhat short
Malalta: short
Malaltega: very short

Does this sound accurate?

That matches up with how I think of things.