What Image Size for Film credits ?

Do not try and design titles for a screen size on which the movie may be watched.

The big question you need to answer is - what pixel size is the movie, to which your title credits will be added?

If it is HD then designing at 1920 x 1080px is correct. If it is UHD then design at 3840 x 2160px, if full 4K use 4096 x 2160 and if full 8K 7680 x 4320.

In short, match the movie pixel size. Making credits any larger will result in them being scaled down when they are added to the movie which may affect and blur any fine noise/detail in your image, any smaller and they will be scaled up and could introduce scaling artifacts. 

Dave

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@Tommaso5C4F wrote:

Now (I've since subscribed to Photoshop 2021), when I set that Image Size, and put the zoom at 100%, the image is very small, smaller than the Photoshop working window. And this is not suitable for a movie, which will be projected on a big screen.

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There is actually nothing wrong. It is a problem of using the wrong method to understand pixel dimensions.

In a theater or on TV, movies are shown so that they fit the size of the available screen. But you are using 100% magnification, which does not mean fit the screen. 100% is the wrong magnification. You should be choosing View > Fit on Screen, to see the design at a normal viewing magnification for the display size you are using.

Actually, if you are designing movie credits for today’s media, you must not lock yourself into a single magnification. You should test your credits for readability at all of the sizes you anticipate the audience will watch the movie. If some will watch on a 27" computer screen, and some will watch on their 6-inch smartphone, or 15-inch laptop display, or their 65" TV screen, or their 8-foot projector screen, then you should make sure your credits read properly at all of those sizes. Not just what 100% magnification looks like on one display size.

You can open multiple windows for the same Photoshop document, set them to different magnifications, and make sure the text is readable on all of them as you edit.

If your question is partly because 100% seems smaller than it used to be, here is a longer explanation:

Are you using a different display or computer than the last time you created a 1920 x 1080 pixel document for movie credits? In the past, many 1920 x 1080 displays were 24 or 27 inches measured diagonally. But today, more 24 to 27 inch displays are 4K, or 3840 x 2160 pixels. That means that along one dimension (height or width), there are twice as many (2x) pixels per inch on a recent 24-27 inch 4K display than there were on older displays.

When you now have twice as many pixels in the same space, those pixels are now half the size as they used to be. Photoshop (like many other image editing progams) defines 100% magnification as one image pixel to one screen pixel. If you are using a display that is 3840 pixels wide (4K), then 1920 x 1080 pixels cannot possibly fill the screen, because 1920 is only half of 3840. On a 4K display, 1920 x 1080 pixels will look like it fills about half the screen height and width, or about 1/4 the area. Or, as you noticed, typically smaller than the Photoshop document window on a 4K or desktop display.

720x480, 1920x1080, and 3840x2160 can each look great "projected on the big screen."

You'll want to find out what the video editing project settings are and then set your image size to match the project frame size.  For example, if your project settings are 1080p23.976, 1920-by-1080 is a good image size; however, if your project is 2160p23.976 then your image size will likely be 3840-2160. 

Either the producer or editor should be able to provide the video editing project settings.  If someone isn't sure, ask for a freeze frame from the full resolution edit or a short video clip from the full resolution edit.  You can open this in Photoshop and then set your image size settings to match.

If you want to get started now and find out later what the project settings are, go with 3840-by-2160 and down-convert or resize later if needed.

You need to know the size in pixels if your screen. I imagine it's a new screen or laptop. What is the size?

What is density and dimensions of your screen? Same as before? If that is print size then percent shouldn't be 100%. Try with preference reset what is general advice for all things weird in Photoshop https://helpx.adobe.com/photoshop/using/preferences.html