The non-subscription (free) accounts (56k dial-up) and the old ADSL HomeSurf account on PlusNet have just 25MB of webspace so I assume you are on one of those. If you wish to get the 250MB of webspace that other home accounts have then you will have to change or upgrade your package. This will depend on what product you currently have. This page will tell you your current product.
If you are on ADSL HomeSurf then you will need to change to one of the new Broadband Home accounts to receive more webspace. See the upgrade guide FAQ on the support section for the costs involved in this.
If you are a pay-as-you-go free account user. You can eithe rupgrade to an unmetered 56k account, a broadband home account or get the Lite enhanced pay-as-you-go package.