Hi @Sofoklis_Gourdoukis to increase the text area simply select the Form Block > Form > Text Field element and head over to the Size settings on the right hand side and change the height to whatever you wish.
Regarding Hitting Enter / Next Line stuff, you can check out this post from the forum… How to disable enter to send message - #2 by StuM
But do consider accessibility and website users that utilise this functionality when submitting forms.