Mortar question

[Dino69]

My sailor course has tiny shinkage cracks. I was a bit enthused to finally be laying down some bricks and filled the gap completely with my fine sand homebrew. I hope it is not a problem for me later.

I had actually planned on using a different size aggregate just for this issue. But it was a nice day and I could not wait to get to work. What size should I use for the gaps on the back of my oven? Is the play sand at Lowes big enough or should I go a bit larger?

Thanks to everyone. I would be lost without you guys.

David