Creating existing pattern or tile for certain size

In more recent versions it is easier to scale a single tile to the size you want:

Make sure your units are set to points or pixels.

Create an empty artboard.

Drag your pattern from the Swatches panel to the artboard.

In the Layers panel locate the path at the bottom of the group that was created by dragging the patten swatch.

Select that path, it should have no fill and stroke.

Choose Object > Artboards > Fit to Selected Art

Now your artboard has the same size as a single pattern tile.

Double click the intersection of the rulers to set the upper left X/Y coordinates to 0/0

To scale it to the size you want:

Select the Artboard Tool (Shift + O)

In the Properties or Control Panel check the Scale Artwork with Artboard option.

Enter the desired width or height with the “Maintain Width and Height proportions” chain symbol checked.

Now you have a tile at the size you want, you can drag it back to the Swatches or export it as a single tile.

Another thing to realize: Often, stock patterns include multiple copies of the pattern within the pattern artwork (which is the case in your sample). You can use a portion of the pattern and get the identical results of using the entire pattern artwork. This may work for the patterns of different sizes you refer to.

morinaga,

I can't force it to fit to that size by changing its dimensions without ruining its visual look.

Indeed: it is impossible to change something from rectangular to square without changing/wrecking the proportions.

If the rectangular tile (only) repeats itself at different widths and heights, it is impossible to make it square, as that would require its repeating itself at identical width/heights.

But presuming vector artwork, what is wrong in uniformly scaling the size so that either width or height becomes 1024 (you may create a new swatch with the desired dimensions) and then let it repeat itself at its own will?

You mean you want your pattern Tile to be 1024 X 1024?

Your example seems to have a square repeating pattern tile.

It should not be too difficult to scale that to 1024.

Make sure your units are set to points or pixels.

Drag your pattern from the Swatches panel to the artboard.

In the Layers panel check the path at the bottom of the group you used created by dragging the the patten swatch.

Select that path, it should have no fill and stroke, and look at the dimensions, if it is a square you can scale that rectangle together with the rest of the group to 1024.

Now you have a 1024 tile, you can drag it back to the Swatches or create an Artboard of the same size.

"Demanded Size"? Is this something new in AI?

If the pattern is already done and cropped to repeat I would just drag it into the "Swatch" panel. It should become a pattern automatically.

If it needs to be cropped you may need to add a non-stroke/fill rectangle behind it before dragging to Swatch Panel.