What are the properties of error vector used in symmetric McEliece cryptosystem?
The weight of the synthetic error vector is $n/2$ where $n$ is the block length, this corresponds to maximum entropy error patterns, with independent probability of error $1/2$ for each bit. 

The error correcting capability of the code should be much lower for good security.

In the original proposal, $n=1024$ and $t=50$ so this should give you some idea. There is a very readable paper here about this cryptosystem. Look for papers by Sendrier, for updated information on parameters and attacks.