I had a need for a large poster recently and here's how I did it.
The requirement was for A0 size (1189mm x 841mm) set at portrait.
In Powerpoint, click design, page setup, and custom size.
Design as a normal slide, but using much larger fonts for text, line widths, etc. Graphics (photos, etc) must have high resolution, as any imperfection will be enlarged
You must then convert to an A0 size .pdf. I use PrimoPdf (free download) and set page size to above size, and resolution to 300dpi.
Resulting .pdf was over 3mb. Emailed in to
www.blueprint.ie (Stillorgan) who printed it within a few minutes. €35 - full colour - excellent.