You can use this CSS code to make the form bigger. Please go to Design > Custom CSS and paste this code. It has the instrusctions if you want to make changes
#mlb2-5625040.ml-form-embedContainer .ml-form-embedWrapper.embedForm {
max-width: 1000px; //Sets the width
width: 100%;
}
To get the mailerlite communicate with your SS site, copy the JS code at the mailerlite and put it into the SS site. Follow these steps
Please go to Settings > Advanced > Code Injections > Header > Paste the code into header. let me know if you may need any further help