Hi Milan,
Sorry for the delay! Right now the forms and subscription pages are in tables with specifically set widths, as you said. You could get around this by going to Embed > Embed as Link > Download File and downloading the raw HTML for your web content. From there, you could change the layout as you like. Please note that we wouldn't be able to provide support for custom code changes like this.
We are also working on creating responsive web content, which we anticipate having available shortly.
Please let us know if you have questions!