Assuming that you use some custom LWC modal component, you could replace the standard classes used with custom ones to remove 50% width constraint and some other which prevents date pickers from working correctly.
If you the following line in your modal html file:
<div class="slds-modal__container">
replace it with
<div class="container">
and add some styles from slds-modal__container
to your custom css file
.container{
position: relative;
transform: translate(0, 0);
transition: transform var(--lwc-durationQuickly,0.1s) linear,opacity var(--lwc-durationQuickly,0.1s) linear;
display: flex;
flex-direction: column;
justify-content: center;
margin: 0 var(--lwc-varSpacingXLarge, spacingXLarge);
height: 100%;
padding: var(--lwc-squareIconLargeBoundary,3rem) 0;
border-radius: var(--lwc-borderRadiusMedium,0.25rem);
}
If you using your LWC inside the standard Quick Action modal, there is no other workaround that redirect to custom component