> vöxtr ( gen - vaxtar)dat.sing vexti - in size at vexti in shape

That's right. On its own, 'vöxtr' can mean "size" or "stature".
It's related to the verb 'vaxa' "to grow" (compare English "to wax
lyrical" and "waxing moon", and the noun "wax").

This is a typical u-stem [
http://www.utexas.edu/cola/depts/lrc/eieol/norol-2-
X.html#Nor02_GP07_02 ], with u-umlaut in the nominative sg. and i-
umlaut in the dative singular.

It might help to look at how the word evolved from the precursor
language.


Proto Norse Old Norse (Old Icelandic)

Singular Singluar

N *wahstuz N vöxtr
A *wahstu A vöxt
D *wahstiu D vexti
G *wahstôz G vaxtar


Plural

N *wahstiuz N vextir
A *wahstunz A vöxtu
D *wahstumz D vöxtum
G *wahstô G vaxta


The regular developments is: wherever /i/ came next, the root vowel
underwent i-umlaut; and wherever /u/ came next, an /a/ in the root
became /ö/ in Old Icelandic. See Lesson 6, 1.1 [
http://www.hi.is/~haukurth/norse/olessons/lesson6.php?colors=0 ].
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Sebastian: What is Nommish?
Patricia: a quality unique to the Llammasery to which our LN
belongs, a strange humility born of pride in achievment, and joy in
helping others

In other words, I just made it up!
_______________________________________________________________

> Hm, I'm still a bit confused. Apparently þat comes from sá, right?

In Zoëga's dictionary it says this is neuter, what is neuter in the
sentence? Possibly you can give more examples of use of þat?


Yes, in the nominative: neuter 'þat', masculine 'sá',
feminine 'sú'. In a sentence like 'þat er þeir sjá' "what they
see", the neuter acts as a sort of default case for any item,
whatever they see, whatever gender it may turn out to be, just as we
say "what" or "that" or "it" in English. Even where the gender of
the thing referred to is not in doubt, neuter may be used in Old
Norse, for example in definitions:


Síðan gerði hann myrkvastofu; þat er þessi heimr, er vér byggjum.
"Then he made a dungeon; that is this world which we inhabit."
('myrkvastofa' "dungeon" = feminine; þat = neuter; 'þessi
heimr' "this world" = masculine).

Þat er ómennska ef maðr gengr með húsum fyrir nenningarleysis sakir
eða ókosta annarra þeira er góðir menn vilja fyrir þeim sökum eigi
hafa þau.
"It is ''perversity'' if a person goes from house to house because
of indolence or other failings which make good men unwilling to have
them."
(A legal definition: 'ómennska' "perversity" = feminine).

Þat er hin þriðja náttúra jarðar, þá er hon er opnuð ok grafin, þá
groer gras á þeiri moldu er efst er á jörðunni.
"That/this/it is the third property of earth, [that] when it is
opened up and dug, grass grows on that soil which is uppermost on
the earth."
(þat = neuter; hin þriðja náttura = feminine; hon = feminine,
referring to 'jörð' "earth".)


Neuter also used as default where the reference is abstract and not
to any particular noun: 'þat er sagt' "it is said". Similarly with
adjectives. Here's a curious quote:

ok er því gott góðu at trúa, en illt er at trúa illu, þótt satt sé
"and so it is good to believe in good things, but bad to believe in
bad things, even if they are true" (!) [
http://narrowshore.blogspot.com/2004/11/tt-satt-s-again-with-
information.html ].

With people, 'hverr' "who" may be used: 'Hverr er sá maðr, er svá er
spurull?' "who is that who is so curious/questioning?" (The 2nd 'er'
here is the relative, the 3rd = "is".) But even when the referrence
is to a noun that is clearly animate and grammatically non-
neuter, 'hvat' may be used together with the genitive plural:

hvat er þat fiska?
"what fish is that" (literally "what [one] of fishes").

hvat er þat manna?
"what man is that", "what sort of a man is that", "who is that"

hvat manna ertu?
"what sort of a man/person" are you", "who [exactly] are you"
(the answer might include any or all of details such as name,
lineage, status/occupation (king, beggar, etc.), place of origin,
country ruled over, etc.)

Also dat. sg. is possible: hvat er þat drykki? "what drink is that"
(what sort of a drink is that). But the genitive constrction seems
to be more common.