Hi i have credit to get a poster print fron Photobox.Size 40x30 inches but it will not accept file over 10m and as a 40x30 by even 72dpi works out at 17.5m How am to solve this as Photobox are not very helpful with this.
The file does not need to be anywhere near as large as 10MB for a print 40x30 inches. 150 pixels per inch or even 100 pixels per inch will be adequate for a large print. So for 40x30 inches you need 6000x4500 pixels or even 4000x3000 pixels. The exact number of pixels is not really relevant as print services, such as Photobox, will resize at runtime to whatever dimensions you have chosen.
Assuming your file is something like that number of pixels, simply save it as a .jpg at quality level 8 in Photoshop or any other photo editor. You will probably get a file somewhere between 1MB and 5MB, depending on the image.