Is there any way - in the default - I can feature database entries in a nice way. I can only find the setting that says show only featured and then anything is gone. And sorting doesn't seem to offer this option of featured on the top neither does the order option. Turning them into a block didn't come out that appealing. So question remains how do my featured clients end up on top of the list/page. If the default settings cannot do it does i.e. Supergrid do this?

To put it on top of the listings, you select “pin”, not “feature”. Featured entries could however be shown in a block for example, as you have mentioned. Using the SuperGrid block template would make that much more appealing – at least as long as all feature entries have a proper Record Image available. 

What size would you recommend a fall back image to be? ?