Step 1: Open Blogger.com and goto Layout>>Edit HTML.
Step 2: Backup your template by downloading it.
After that click on Expand Widget Templates and search for
.post blockquote
{You will get a code like this :
.post blockquote
{margin:1em 20px;
border:1px dashed #40a9c9;
background: #e7f9ff ;
padding: 8px 8px 8px 65px;}
.post blockquote p {margin:.75em 0;}
To customize it you will have to change the following 3 things :
1) Margin : Tells how much size you want.
How?
Margin : To increase the size of blockquote you can increase the pixels i.e from 20px you can make it 25px or 30px
2) Border : Tells what kind of borders you want for blockquote i.e you want dotted borders,simple borders etc.
How?
Borders : You can customize borders.You can make it dotted like I have done.To make it dotted just replace the border code with this one border:1px dashed #40a9c9; You can replace dashed with solid, dotted,none,hidden,,doublegroove, ridge, inset, and outset.
More: http://www.w3schools.com/htmldom/prop_style_borderbottom.asp
3) Background : Tells the background image you have set for blockquote.It can be 2 quotes like this .You can customize all these very easily,so lets take it one by one.
How?
You can change the background colour to any colour you want.Just have a look at this HTML colour chart.You can also make borders thick by increasing pixels.Background : To change the background,you have to change the code to
background: #FFF098 url(your-image-url) no-repeat;Background : You can use other images also.You can find alot of image in Google.Just search for blackquote in images.google.com