Choosing a Harddisk is rather one of the most easiest task compared to all the procedures involved in building or upgrading a computer. But you would need to have atleast the slightest of the ideas of the Sizes of the Data you could be storing in your desktop. Though the harddisks can be upgraded at any point in time, it is not advisable to go for a smaller size indending to upgrade it in a short term which would increase your cost of owner ship of the computer. A large size of this component could free you from the hassle to free your harddrive everytime you want to store however but a low capacity is advisable in case you have no plans to store a large amount of data which would also be low on your pocket.
You would be able to estimate the size of the data that you would be storing on your desktop in long term only when you have the idea as to what would be the space occupied by different file formats. For example, if you are a multimedia person, you would require enormously sized harddisk atleast a 320 GB HDD or 500 GB HDD as videos and images occupy huge space in the disks. If you are looking for entertainment too, a 250 gb to 320GB disk should be prudent that will fetch you enough space for storing your movies, images and more. But if it’s a case of student just looking to store assignments mostly done in Microsoft Office and like, a 160 GB to 250 GB could be recommended. Similarly for the professional just looking to store their presentations and like which are not huge in size , the afore mentioned size would do.
Remember , some softwares occupy a huge space too, so make sure you plan accordingly in case you are looking to download these kind of softwares. Other things to look for when buying one is the Interface and RPM among others.