Just add to what Wardy has said; If you use illustrator or other vector software, you will be able to blow the images up as large as you like. If you use photoshop you will need to determine the final maximum size when you first set your illustration up. You can make pixel-based images smaller, but if you increase their side from the original they degrade.
Of course, but as they said they were new to this, I thought I’d stick to basic rules of thumb, as it is going to be a steep enough learning curve for them as it is if they have never done anything like this in the past. We’ve all seen it before, where clients supply images that are fine at the size of a peanut, but they want them the size of a house. Too easy a trap to fall into, especially if you are a newbie.Not entirely accurate - you can use Vector Shapes, Vector Masks and Text layers in photoshop and retain output of the vectors - especially saving it as Photoshop PDF - or printing directly from the PSD via Photoshop.
Of course, but as they said they were new to this, I thought I’d stick to basic rules of thumb, as it is going to be a steep enough learning curve for them as it is if they have never done anything like this in the past. We’ve all seen it before, where clients supply images that are fine at the size of a peanut, but they want them the size of a house. Too easy a trap to fall into, especially if you are a newbie.