Is it possible to show full content of the post when there is only one post in a grid?

not sure how you’re actually displaying your posts whether it’s a custom query and which page your doing your grid, for example a category page. Here’s a little snippet that may help you.

//get current category page
$category = get_the_category();

//get current category page ID
$category_id = $category->cat_ID;

//set arguments for your desired posts output
$args = array(
'numberposts'     => 999,
'offset'          => 0,
'category'        => $category_id,
'order'           => 'DESC',
'post_type'       => 'post', //if your using a custom post type enter it here
'post_status'     => 'publish' );

$all_posts = get_posts($args);

//get amount of posts
$post_amount = count($all_posts);

//now we know the amount of posts you have in this category, we can do a conditional.

if($post_amount == 1){
//show all your content.
}

I haven’t tested this but I’m pretty sure the logic is correct.