big size files!!

can anyone help me I keep on creating files that are 1gb big with a simple 400x100 size using RGB colouring and a plain white background. Has anyone ever experinanced this before? would someone please be able to help me? Hope I've posted in the right place.
regards,
Andrew

That 400x100 in pixels, or inches?

Mac

thats just cm

That's quite large. Unless you're creating 1:1 world maps. Please create the following:
100x100 pixels RGB 72 DPI. Then please tell us what size that is. Rob

thats just cm

There's your answer.
400x100 cm at 190ppi (the largest can create under 30,000 pixel width) is about 640MB. Add a layer or three and you'd be right up around the 1GB size you report.
There's a reason you need such a large physical sized image?

Mac

ok done that what scale should I use (mm, cm, inch).
And should I use pixel/inch or pixel/cm
regards,
Andrew

ok done that what scale should I use (mm, cm, inch).

The one you are most comfortable in or that makes the most sense

And should I use pixel/inch or pixel/cm

The same.

Not knowing what your project is and how it will be printed, there is no single correct answer, but a good rule of thumb for print work (unless you are doing large format) is to work at 300 pixels per inch (ppi) or 120 pixels per cm.

ok done that what scale should I use (mm, cm, inch).
And should I use pixel/inch or pixel/cm
regards,
Andrew

Entirely up to you.
You need to design the output size you actually need for a project, of course.
We Mericuns tend to think in inches, pixels/inch FWIW.
But it doesn't matter as far as an image is concerned; that is, a 400x100cm image at 74.8 pixels/cm is 157.4 x 39.3 inches at 190 pixels/inch.
Both images are the same physical output size and both contain the same number of total pixels 29921 x 7480.
(Photoshop has a 30,000 pixel limit on any dimension, unless you are using the alternate file format in the new CS version).

Mac

alright thanks for that now the file is actually turning out a bit smaller :) Thanks