By default, Mailwizz’s DKIM key size is 2048 bit which should be supported by all DNS platforms and you should not have any issue adding it as a TXT record.
However, some providers don’t support 2048 bit keys, only 1024, which means when you paste the 2048 bit key, it gets truncated.
We suggest reaching out to your service provider and ask them to support 2048 bit keys.
If you indeed have no other option, Mailwizz offers you the possibility to switch to 1024 bit keys.
To do this, you need to add a new app param as explained here.
The param you have to enter is:
'email.custom.dkim.key.size' => 1024,
so the param name is email.custom.dkim.key.size and the param value is 1024.