Firstly you need to know which html renderer will render your html code. Visit this site: http://www.campaignmonitor.com/css/ , after this you can decide which solution is better.
1) Use inline-css for your table tag like this:
<table style="height:50px;"></table>`
2) Use an internal style sheet for your table tags like this:
<head>
<style>
table { height:50px; }
p { color:blue; }
</style>
</head>
3) Use an external style sheet like this and give a height value for the table like this:
<head>
<link rel="stylesheet" type="text/css" href="mystyle.css">
</head>
Or you can give a value for the td elements and then the table will automatically figure out it's size.
Example: if you work on an html email you can't use external css. You need to solve it with inline-css (solution 1 above). If it's not supported you are still able to do it in the td tags.
