様々な大きさの画像を自動で指定した大きさにするにはどうしたらよいですか？

そのような場合条件付きアクションで出来ますね。

1. 画像のフィットで長辺を500に合わせる。

2. センター合わせでカンバスサイズを500x300または300x500にする。

3.  保存。

こんな感じでどうでしょう。

You would need to use Photoshop Scripting to do that.  In my Crafting Action package I supply a Photoshop Plug-in script that you can record in your action.  That  will enable you the make the aspect ration crop you need to make to change your images aspect.  AspectRatioSelection plugin.  However, the Aspect Ratio selection will be for the images orientation  It will not set a portrait selection for a landscape image. For too much image content would be lost if you made that crop.   So if the two numbers you recorded in the plugin step are 2 and 3 the selection set for a landscape image would be 3:2 for a portait image the selection set would be 2:3.

Crafting Actions Package UPDATED Aug 10, 2014 Added Conditional Action steps to Action Palette Tips.
Contains

• Action Actions Palette Tips.txt
• Action Creation Guidelines.txt
• Action Dealing with Image Size.txt
• Action Enhanced via Scripted Photoshop Functions.txt
• CraftedActions.atn Sample Action set includes an example Watermarking action
• Sample Actions.txt Photoshop CraftedActions set saved as a text file.
• More then a dozen Scripts for use in actions

Example
Download

Step 1 File Automate Aspect Ratio Selection:   Number, Number

Step 2 Image Crop

Step 3 File Automate Fit Image:   larger side number of pixels, larger side number of pixels

Step 4 Image size Resample NOT CHECKED set print PPI resolution

Fit Image is an Adobe supplied plugin Script.