A 'bad' definition for the cardinality of a set My set theory notes state that the following is a 'bad' definition for the cardinality of a set $x:$ $|x|=\{y:y\approx x\}$ $(y\approx x\  \text{iff} \ \exists\  \text{a bijection}\  f:x\rightarrow y )$
The reason this is a 'bad' definition is since if $x\neq \emptyset$ then $|x|$ is a proper class and I am asked to prove this. 
For the moment, let's consider $x$ to be finite. Say $|x|=n$ as $x$ contains a finite number of elements. 
But then $x\approx \{n+1,...,n+n\}$ as $|\{n+1,...,n+n\}|=n$ and we can 'keep going up' in this way such that  $\bigcup On\subseteq|x|$ and since $\bigcup On = On$  and $On$ is a proper class we must have $|x|$ being a proper class as well.
This argument is certainly not rigourous and I am unfortunately very much stumped on what to do if $x$ not finite.
$\underline{\text{Please correct me if $\ $} \bigcup On \neq On}$ 
Any feedback is very much appreciated. 

Here is a possibly easier approach: show that if $x$ is not empty, then for every $a$ in the universe, there is some $x_a$ such that $a\in x_a$ and $x_a\approx x$.
If $a\in x$, then $x_a=x$. Otherwise, fix some $y\in x$, and consider $(x\setminus\{y\})\cup\{a\}$ as your $x_a$. Now it follows that $\bigcup |x|$ is everything. And everything is not a set.
