to answer your first question, Katerina, insert this into the style.css document of your theme:
/* Modify recipe cards */
.blog-yumprint-recipe {
max-width: 600px;
margin-right: auto;
margin-left: auto;
}
the max-width number is what you change to determine the largest size of the recipe card. this number can also be a percentage.
the margin-right and margin-left, when set as auto, center the card on the page.