When creating flyers in Photoshop, the canvas needs to be created at the full size, plus bleed, at 300dpi.
So an A5 flyer needs to be 154x216mm and 300dpi. That's the A5 size (148x210mm) plus the 3mm bleed on all sides.
Bleed is explained here.
It is also much better to supply as CMYK so we don't need to convert it, so the colours will stay closer to the original design, and we use the colour profile for coated paper "Coated FOGRA39"
We have templates of common formats (including Photoshop) for download here.
Please see below for a PSD setup for an A5 flyer as an example.
You can return to the artwork guidelines page here.