Please find the CSS for editing the Icon Size on the Activity page. Add the CSS in Edwiser RemUI setting > General setting > Custom CSS section

#page-course-view-remuiformat .wdm-section-wrapper .single-card-container .single-card .wdm-single-activity-title.nosummary img {
    height: 100px;
    width: 100px;
}


Note: This is applicable only when you have not added a description to the activity.