Hi there,
Yes, it is possible to change the sizes of widget review item thumbnails, and text. You would need to add this CSS snippet:
.cwp-popular-review .cwp_rev_image {
width: 70px; // change 70px with the size that you want for thumbnail
}
.cwp-popular-review .wppr-col {
font-size: 25px; // change 25px with the size that you want for text
}
You can add this code by going to Dashboard > Appearance > Customizer > Additional CSS.
Let me know if this worked for you. Thanks.