Greetings and welcome to the Elfsight family, @J_MR2 
To display only 1 post without a Load More button, feel free to use this CSS code - Remove Load More button.
As for the widget’s size, you can control the width and height on the Layout tab of your widget’s settings. Do these options work for you?
If the widget isn’t centered on your website or you have issues with the responsiveness, please send me a link to the page, where your widget is installed. I’ll gladly look into this for you 