Do not use more than one style attribute. Just seperate styles in the style attribute with ;
It is a block of inline CSS, so think of this as you would do CSS in a separate stylesheet.
So in this case its:
style="height:100px;width:100px;"
You can use this for any CSS style, so if you wanted to change the colour of the text to white:
style="height:100px;width:100px;color:#ffffff"
and so on.
However, it is worth using inline CSS sparingly, as it can make code less manageable in future. Using an external stylesheet may be a better option for this. It depends really on your requirements. Inline CSS does make for quicker coding.