For Illustrator....
Create a square and circle, centered...

Remove the stroke and fill from the square and move the square behind all other objects....

Select all that and drag it to the Swatches Panel to create a pattern swatch.
Draw a new rectangle and apply the new pattern you created as a fill...

If it's not the size you want... choose Object > Transform > Scale...
from the menu.
Tick the Transform pattern box. This should UNtick the Transform Objects
box. If it doesn't untick the transform objects. Then just enter a percentage of reduction to resize the pattern....

@Cai pointed out that the dots are staggered... easily done...
Double-click the new pattern in the Swatches Panel. When the Pattern Options
Panel pops open, select Brick By Row
from the Tile Type
drop down, then just exit pattern editor choosing to save the edits.

Illustrator screenshots from AICS6 because I'm not a big fan of CC.... but it's the same basic thing.
--- Ooops original answer failed to realize this was an AI question.... below is Photoshop... -----
It's just a dot....

You can create the pattern then apply it as a pattern overlay then adjust scale to your liking...