Example code
Step 1
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Change posts thumbnail size on archive pages by post type
In the following example you can see how to change posts featured image size on posts archive pages.
XTRA
Some available sizes
thumbnail
medium
full
codevz_360_320
codevz_600_600
codevz_1200_200
codevz_1200_500
codevz_600_1000
codevz_600_9999
XTRA
How and where should I add this PHP codes?
Best solution is to add custom actions and filters in Child Theme functions.php file. But also you can create your own plugin and use this codes in your plugin.
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