First, I think you would really benefit from trying out Firebug for Firefox: http://getfirebug.com . Generally, when we go to modify a css class (like for this) we'll use Firebug or something like it to "Inspect" the css.
So for what you are looking to change, lets take a look at: http://www.kriesi.at/themes/angular/
The main page has the portfolio elements we are looking to change. I'll open the site in Firefox and then right click on the little squirrel in the Happy Holidays portfolio item and select Inspect element with firebug.
From the inspection window, you can see the img with its attributes, the canvas class beneath it to handle the greyscale-image etc. What we are looking for is what is effecting the border of the item. Moving up through the css I eventually find that the div class="inner-entry" is responsible for the border around the items. In this case, its actually a box shadow giving the effect of a border.
So what you would put into your custom.css or Quick CSS would be:
We get rid of the box shadow so that you can add a border then add attributes for the things we want to happen. Sorry if this comes off a little to pedagogical but I've noticed you are really digging in to customizing the theme and I thought this would help :)
The title size:
Box size is done with the portfolio layouts (either 4,3,2,1). Once you start digging into changing them, you will probably run into issues with the display of the theme with smaller platforms.
You can also apply a background-image to the header class.