page.css

div .LARGE {
    background : red;
}
div .MEDIUM {
    background : green;
}
div .SMALL {
    background : blue;
}

page.html

<template>
    <h1>lwc-page</h1>
    <div class={flexipageRegionWidth}>
            {flexipageRegionWidth}
    <c-slots>
        <span slot="title">toast</span>
        <span slot="body">
            <c-toast></c-toast> 
        </span>
    </c-slots>             
    <c-slots>
        <span slot="title">apex</span>
        <span slot="body">
            <c-apex></c-apex> 
        </span>
    </c-slots>      
    <c-slots>
        <span slot="title">wire service</span>
        <span slot="body">
            <c-wire record-id="001B000000D7NopIAF"></c-wire> 
        </span>
    </c-slots>     
    <c-slots if:false={standalone}>
        <span slot="title">pubsub demo</span>
        <span slot="body">
            <c-pubsubleft></c-pubsubleft> ===
            <c-pubsubright></c-pubsubright>
        </span>
    </c-slots>
    <c-slots>
        <span slot="title">navigation demo</span>
        <span slot="body">
            <c-nav></c-nav>
        </span>
    </c-slots>   
    <c-slots>
        <span slot="title">lightning record form</span>
        <span slot="body">
            <c-lrf record-id="001B000000D7NopIAF"></c-lrf>
        </span>
    </c-slots>   
    <c-slots>
        <span slot="title">lightning record view form</span>
        <span slot="body">
            <c-lrvf record-id="001B000000D7NopIAF"></c-lrvf> 
        </span>
    </c-slots>   
    </div>
</template>

page.js

import { LightningElement, api } from 'lwc';

export default class Page extends LightningElement {
    @api flexipageRegionWidth; 
    @api standalone = false;
}

page.js-meta.xml

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="page">
    <apiVersion>45.0</apiVersion>
    <isExposed>true</isExposed>
      <targets>
      <target>lightning__RecordPage</target>
      <target>lightning__AppPage</target>
      <target>lightning__HomePage</target>
  </targets>
</LightningComponentBundle>

page.svg