Thumnail on index.php, full-size on post page

So, when I create a post and I add the excerpt <!—more—> feature to the post, and I have an image in the post, the image is displayed at the size I determined when I inserted it.

I would like to be able to insert a full-size image into the post while I create it, but place a condition that says, “if the post has a “more” and it shows up on the posts page, thumbnail the image on that page.

As it is now, it’s either one size or the other when I insert, and as a result, the images, when aligned left or right, overlap the (read more…) boundary into the next post title.

Is there a condition that I can use such as `<?php if this page is index.php or the posts page

{ do this };?>`

???