How to change product image size in Magento 2

[Whaaaaat]

In the gird view of a category I want to reduce the number of products per ROW from 4 to 3, and I want to increase the image size there ... how can I do that?

[David_]

The image re-size dimensions can be changed in <theme>/etc/view.xml, for the category listing pages you would need to change the <image id="category_page_grid" type="small_image"> dimensions.

How have you made your theme? Do you use Blank as the parent?

[NWDThemes]

You can also specify image dimensions directly in template file like this:

<?php
/**
* @var $block \Magento\Catalog\Block\Product\Widget\NewWidget
*/
$image = 'new_products_content_widget_grid';
$items = $block->getProductCollection()->getItems();
$width = 100;
$height = 100;
foreach ($items as $_item) {
    $resizedUrl = $block->resizeImage($_item, $image , $width, $height)->getUrl();
    echo '<img src="'.$resizedUrl .'" alt="alt text" />';
}

More samples here - https://nwdthemes.com/2017/12/19/magento-2-product-image-size/