I am trying to resize the image. Changes are made to the tinyMCE processor, but changes are not displayed at the front of the website.

Is there a solution to the problem?

@kriskoyk
Not quite sure what you mean by changing image size

On a page I upload an image which is too large and I want to shrink it but the resizing system does not work. To be precise, in the bender theme, the size correction I make appears, but in the sigma theme, its original dimension always appears.

I change the SIGMA theme in style.css this:

/* RESET */
fieldset{border:none;display:inline-block;padding:0;margin:0;width:100%;}html,input,select,textarea,button{font-family:'Nunito',sans-serif;}div{margin:0;padding:0;}html{color:#222;box-sizing:border-box;font-size:14px;line-height:18px;cursor:default;-moz-tab-size:4;tab-size:4;-ms-text-size-adjust:100%;-webkit-text-size-adjust:100%;word-break:break-word;}*,*:before,*:after{box-sizing:inherit}body{display:block;margin:0;font-weight:normal;width:100%;}p,ol,ul{display:inline-block;margin:0;padding:0;font-weight:normal;width:100%;}ol,ul{list-style:none}img{max-width:100%;width:auto;height:auto}
body, html {direction:ltr;}

 with this:

 /* RESET */
fieldset{border:none;display:inline-block;padding:0;margin:0;width:100%;}html,input,select,textarea,button{font-family:'Nunito',sans-serif;}div{margin:0;padding:0;}html{color:#222;box-sizing:border-box;font-size:14px;line-height:18px;cursor:default;-moz-tab-size:4;tab-size:4;-ms-text-size-adjust:100%;-webkit-text-size-adjust:100%;word-break:break-word;}*,*:before,*:after{box-sizing:inherit}body{display:block;margin:0;font-weight:normal;width:100%;}p,ol,ul{display:inline-block;margin:0;padding:0;font-weight:normal;width:100%;}ol,ul{list-style:none}img{max-width:100%;width:auto\9;height:auto}
body, html {direction:ltr;}

 and the changes I make to the image size appear normally, but I do not know if the method is correct.

@kriskoyk
I would recommend you:
1) provide url
2) provide steps to reproduce problem
3) provide screenshot

@kriskoyk
Not sure what is wrong there, image is properly shown based on it's width/height.

The images are not the same. The first is 320x240 and the one that appears in the sigma theme is 900x675.
That is, while I reduced the photo to 320x240, the photo appeared in the sigma 900x675 theme.

The same problem appears in your demo

https://demo.osclasspoint.com/oc-admin/index.php?page=pages

https://demo.osclasspoint.com/test123-p26

@kriskoyk
As far as my eyes can see, image is properly shown. No overflow, proper aspect ratio.

Τhis image is 320x240?

https://demo.osclasspoint.com/test123-p26

Theme contain auto corrective style for images, you will have to disable this on static page. Problem is that tinymce put size as html attributes
img {
    max-width: 100%;
    width: auto;
    height: auto;
}


Is this the file I need to fix?

oc-content\themes\sigma\css\style.css

/* RESET */
fieldset{border:none;display:inline-block;padding:0;margin:0;width:100%;}html,input,select,textarea,button{font-family:'Nunito',sans-serif;}div{margin:0;padding:0;}html{color:#222;box-sizing:border-box;font-size:14px;line-height:18px;cursor:default;-moz-tab-size:4;tab-size:4;-ms-text-size-adjust:100%;-webkit-text-size-adjust:100%;word-break:break-word;}*,*:before,*:after{box-sizing:inherit}body{display:block;margin:0;font-weight:normal;width:100%;}p,ol,ul{display:inline-block;margin:0;padding:0;font-weight:normal;width:100%;}ol,ul{list-style:none}img {max-width: 100%;width: auto;height: auto;}
body, html {direction:ltr;}

@kriskoyk
Correct.