Working codebase using the CSS from SLDS. Try not to use custom divs and pixels based height/width will not work well in all screen types
A side note - You should start using LWC components instead of aura. Soon aura will be outdated in the Salesforce ecosystem. and LWC gels with existing builds where aura is used extensively

<aura:component implements="force:hasSObjectName,flexipage:availableForRecordHome,force:hasRecordId,force:appHostable,flexipage:availableForAllPageTypes,force:lightningQuickActionWithoutHeader"
access="global"
>
<section role="dialog" tabindex="-1" aria-labelledby="modal-heading-01" aria-modal="true" aria-describedby="modal-content-id-1" class="slds-modal slds-fade-in-open">
<div class="slds-modal__container">
<!-- Modal/Popup Box LWC header here -->
<header class="slds-modal__header">
<button class="slds-button slds-button_icon slds-modal__close slds-button_icon-inverse" title="Close">
<lightning:Icon iconname="utility:close"
alternativetext="close"
variant="inverse"
size="small" ></lightning:Icon>
<span class="slds-assistive-text">Close</span>
</button>
<h2 id="modal-heading-01" class="slds-text-heading_medium slds-hyphenate">Custom Popoup</h2>
</header>
<!-- Modal/Popup Box LWC body starts here -->
<div class="slds-modal__content slds-p-around_medium" id="modal-content-id-1">
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
<p><b>Modals/Popup</b></p>
</div>
<!-- Modal/Popup Box LWC footer starts here -->
<footer class="slds-modal__footer">
<button class="slds-button slds-button_neutral" title="Cancel">Cancel</button>
<button class="slds-button slds-button_brand" title="OK">OK</button>
</footer>
</div>
</section>
<!-- <div class="modal-footer slds-modal__footer slds-size_1-of-1">
<lightning:button
aura:id="buttonCancel"
variant="Neutral"
class="slds-button"
label="Cancel"
onclick="{!c.handleCancel}"
/>
<lightning:button
aura:id="buttonSend"
variant="Brand"
class="slds-button"
label="Save"
onclick="{!c.handleSave}"
/>
</div> -->
<aura:html tag="style">
.cuf-content {
padding: 0 0rem !important;
} .slds-p-around--medium {
padding: 0rem !important;
}
.slds-modal__content {
overflow: initial !important;
height: 99% !important;
max-height: 99% !important;
}
.slds-modal__container{
min-width: 100% !important;
width:99% !important;
padding-top: 35px!important;
padding-bottom: 35px!important;
overflow-y: scroll!important;
}
</aura:html>
</aura:component>