I am new to retouching, "very new" so I hope these are not dumb questions, but the only way to know is to ask:

1. When you are retouching a photo that will ultimately be printed for a client (fashion, advertisement, flyer, etc.,) do you retouch in RGB or CMYK? And why? I know that RGB offers more colors than CMYK, but professional printing is done using CMYK, so I was wondering if it ultimately makes a difference in the print how the photo is retouched? If you are just printing something at KINKOs or something does it matter what mode you use?

2. When you are working on a file for a client, how big is the final size and resolution? Does the file come to you at the right size and resolution or do you resize it at the end?

3. If you are using a low-end digital camera, how do increase the size of the photo to say poster print size without affecting the quality?

Bella, welcome to RetouchPRO!
1. Retouch in whatever color space works best for you and convert to your output space when you are done. Each of the color spaces has distinct advantages. In addition to RGB and CMYK, youn should experiment with LAB.
As far as output, most ink jet printers and the printers you find at Kinkos prefer or require RGB files. Their s/w makes more accurate conversion. Commercial printers that use 3 or 4 color printing presses require CMYK separation.

2. Generally, print quality (for photograpic images on photographic paper) is considered 300 DPI. Magazines and newsprint are lower. Ideally if you want to print an 8x10 photo, the image size should be 2400 x 3000 pixels regardless of how it is formatted (8 x 10 x 300 = 4 x 5 x 600 = 16 x 20 x 150...etc).
Clients present you with all kinds of files in all kinds of sizes. Often they are totally unaware of the technical details and you have to request resubmissions if the quality is too poor for the job required.
If you resize a file without resampling it (as in the formulae above. I.E. you have the same number of pixels but you just rearrange them differently) then you do not lose any image quality. If however you resample the image by enlarging it, then you are manufacturing new pixels by extrapolation and you will lose quality. The loss will be proportional to the % of resizing. And you must always remember the addage "Garbage in = Garbage out". If the original file is of low resolution and contains a high degree of JPG artifacts from having been overcompressed, then the up-sized image will look as bad or worse.

3. Notwithstanding the above comments in 2, if you have an image which contains 1 million pixels or more, you can sucessfully enlarge it without noticeable quality loss (to the human eye at reasonable manification). All image editing programs like Photoshop will perform the task reasonably well. Then there are specialized pluggins and applications like Genuine Fractals which are designed to make huge enlargements without any noticeable quality loss.

Regards, Murray

Thanks Murray, though I am a little confused about the resolution formula (8 x 10 x 300 = 4 x 5 x 600 = 16 x 20 x 150...etc). Can you explain it for me in a little more detail?

Thanks Murray, though I am a little confused about the resolution formula (8 x 10 x 300 = 4 x 5 x 600 = 16 x 20 x 150...etc). Can you explain it for me in a little more detail?
If you want to print an image in 8x10 it should be 2400 x 3000 pixels. If the original image is 4 x 5 then you need a resolution of 600ppi, it its 16 x 20 then you only need a resolution of 150 ppi to achieve the same resolution at 8 x10

HTH