I'm trying to make a brush for making stripes, but the Time/Size setting is not working as expected. The Time/Size setting doesn't have any effect on the start of the brush stroke.

Picture of the Brush Editor:
https://i.imgur.com/0iyjE2m.png

I'm using Krita 5.0.0 on Windows 10

You have the duration set to 0.2 seconds which is a very short time.
I think that you're painting those strokes too slowly.
Towards the end of the 0.2 second period, the brush size will decrease and its 'footprint' will shrink so that its dabs stay within the earlier large dab.
I tried those settings and if I paint a stroke quickly then I get the expected effect.
It may be easier to close the brush editor window and try it on the canvas, which has a larger area.