OK, i might have an answer to my own question.

By defining a MaximumWidth for the block control i was preventing the method AdjustWidthOfBlocksToFillRow in SitePropertyContentAreaControls.cs from exiting the while loop because the block width couldn't be increased.  So i have modified the method as follows (Yes this is a bit of a hack and i'm looking to improve it).

        private void AdjustWidthOfBlocksToFillRow(IList<ContentRenderer> group)
        {
            int count = 0;
            while (RowWidth - group.Sum(b => ((IBlockControl)b.CurrentControl).Width) > 0 && count < 12) // While there is space left
            {
                foreach (var block in group.Select(g => g.CurrentControl).Cast<IBlockControl>()) // Increase size of blocks in the row
                {
                    if (block.Width + 1 <= block.MaximumWidth) // Do not exceed block maximum size
                    {
                        block.Width++;
                    }

                    if (RowWidth - group.Sum(b => ((IBlockControl)b.CurrentControl).Width) == 0)
                    {
                        // Row is filled

                        break;
                    }
                }
                count++;
            }
        }

We use pure Html/Css for this. Then based on the model you could add specific CSS classes in your view.

Greetings, Martijn