Etydhv

I am using the excellent tikz-network package written by Jürgen Hackl.
I am trying to change shape of some nodes to be an ellipse, but I can't figure out how to. From the manual, tikz-network:

With the option shape the shape of the vertex can be modified. Thereby the shapes implemented in TikZ can be used, including: circle, rectangle, diamond, trapezium, semicircle, isosceles triangle, ...

But when I compile this code:

documentclassarticle
usepackagetikz-network
begindocument

begintikzpicture
 Vertex[color=white, x=-1, y=1, label=A]A
 Vertex[color=white, x=1, y=1, label=B, shape=rectangle]B
 Vertex[color=white, x=0, y=0, label=2, shape=ellipse]2
 Edge[lw=1, Direct](A)(2)
 Edge[lw=1, Direct](B)(2)
endtikzpicture

enddocument

all I get is this

while I would expect the shape of node 2 to be an ellipse, not a circle.

Actually, your shape is an ellipse. It just happens to be the same height and width, because those are the default settings. You can give extra TikZ options using the style option of Vertex.

documentclassarticle
usepackagetikz-network
begindocument

begintikzpicture
 Vertex[color=white, x=-1, y=1, label=A]A
 Vertex[color=white, x=1, y=1, label=B, shape=rectangle]B
 Vertex[color=white, x=0, y=0, label=2, shape=ellipse, style=minimum width=2cm]2
 Edge[lw=1, Direct](A)(2)
 Edge[lw=1, Direct](B)(2)
endtikzpicture

enddocument

Let me start by spelling out your and and Joule V's observation that the text will just overshoot if is longer than the package expects it to be.

documentclassarticle
usepackagetikz-network
begindocument

begintikzpicture
 Vertex[color=white, x=-1, y=1, label=A]A
 Vertex[color=white, x=1, y=1, label=B, shape=rectangle]B
 Vertex[color=white, x=0, y=0, label=2222222, shape=ellipse]2
 Edge[lw=1, Direct](A)(2)
 Edge[lw=1, Direct](B)(2)
endtikzpicture

enddocument

Needless to say that this looks a bit suboptimal. The internal reason why that happens is that the text is not the node contents, as one may naively suspect/hope, but, as the label indicates, a label. This problem can be solved as follows:

documentclassarticle
usepackagetikz-network
makeatletter
tikzsetnetwork x offset/.initial=1ex,network y offset/.initial=1ex,
adjust size/.style=minimum width=width("vertex@Label")+2*pgfkeysvalueof/tikz/network x offset,
minimum height=height("vertex@Label")+2*pgfkeysvalueof/tikz/network y offset
makeatother
begindocument

begintikzpicture
 Vertex[color=white, x=-1, y=1, label=A]A
 Vertex[color=white, x=1, y=1, label=B, shape=rectangle]B
 tikzsetevery label/.append style=
 Vertex[color=white, x=0, y=0, label=2222222, shape=ellipse,
 style=adjust size]2
 Edge[lw=1, Direct](A)(2)
 Edge[lw=1, Direct](B)(2)
endtikzpicture
enddocument

So all you need to do is to add style=adjust size. As a side effect, this will then really become an ellipse if, as suggested by JouleV, once you insert a wider (or higher) text. The keys network x offset and network y offset can be thought of as the analogues of the ordinary pgf keys inner xsep and inner ysep.