I made a catalogue of specimens in a museum using HTML. when I view the files on my laptop, it can be seen properly. But if it is viewed in another pc the sizes of images and text are very huge.

Ovious is each pc has different resolution, so preferably you should build site for 800x600.
Another thing sometimes our pc's play games with us especially when you often overwrite your work in dreamweaver something on the back of document get mix up and is displayed ok on your pc, however seen on other is complitely different. If you wish put here full code on we will look at it.

body {
background-color: #FFFFCC;
background-image: url(background.png);
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 12px;
line-height: 24px;
color: #336699;
}

h1{
font-family: Times New Roman, Times, serif;
color: #006666; font-weight: bolder; font-size: 40px;
}
h3{
font-family: Times New Roman, Times, serif;
color: #006666; font-weight: bolder; font-size: 22px
}
td, th {
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 12px;
line-height: 24px;
color: #333333;
font-weight: bolder;
}

a {
font-size: 12px;
color: #336600;
font-family: Arial, Helvetica, sans-serif;

}

.title {
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 18px;
line-height: 20px;
background-color: #99CC00; color: #003333;}

.subtitle {
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 16px;
line-height: 30px;
color: #003300;}

.header {
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 20px;
background-color: #669900;
color: #FFFF33;
}

.nav {
font-family: "Times New Roman", Times, serif;
font-size: 14px;
font-weight: bold;
background-color: #CCFF99;
color: #333399;
text-decoration: underline blink;

}

.navLink {
font-family: Arial, Helvetica, sans-serif;
font-size: 14px;
font-weight: bold;
background-color: #DEDECA;
}

.sidebar {
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 10px;
line-height: 14px;
padding: 3px;
background-color: #FFFFCC;
color: #003333;
}

.sidebarHeader {
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 14px;
line-height: 18px;
color: #003300;
background-color: #FFFF66;
}

.sidebarFooter {
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size: 12px;
line-height: 18px;
background-color: #FFFF99;
font-style: italic;
}

.footer {
font-family: Arial, Helvetica, sans-serif;
font-size: 14px;
font-weight: bold;
line-height: 22px;
color: #333333;
background-color: #CCFF99;
}

.legal {
font-family: Arial, Helvetica, sans-serif;
font-size: 9px;
color: #333333;
}

.box1 {
border-width: thin;
border-color: #99FF99 #003300 #003300 #99FF99;
border-style: double;
}

.promo {
font-family: "Times New Roman", Times, serif;
color: #000033;
}

.titlebar {
font-family: "Times New Roman", Times, serif;
font-size: 9px;
color: #FFFFFF;
background-color: #336699;
}

.dingbat {
font-family: Arial, Helvetica, sans-serif;
background-color: #FFFFFF;
color: #660000; font-weight: bolder; font-size: medium;
}

input.big {
width: 100px;
}

input.small {
width: 50px;
}

This is the css file for the site. It can be seen clearly in 1024*768. Now to what pixels should I change the font size so that it will work properly in 800*600.