Salesforce record size overview

Estimated storage for records by object type:

Leads -- 2 KB
Contacts -- 2 KB
Accounts -- 2 KB
Person Accounts - 4 KB
Opportunities -- 2 KB
Forecasts -- 2 KB
Events -- 2 KB
Tasks -- 2 KB
Cases -- 2 KB
Case Team Member -- 2 KB
Solutions -- 2 KB
Notes -- 2 KB
Custom Reports -- 2 KB
Campaigns -- 8 KB
Campaign Members -- 1 KB
Contracts -- 2 KB
Google Docs -- 2 KB
Quotes -- 2 KB
Quote Line Items -- 2 KB****
Orders -- 2 KB****
Work Order Line Items -- 2 KB****
Tags: unique tags -- 2 KB
Custom Objects -- 2 KB
Quote Template Rich Text Data -- 2 KB
Articles -- 4 KB*
Email Message -- 2 KB**
Person Accounts -- 4 KB***

• *May vary based on the content of rich text area (RTA) fields. Images from (RTA) are stored in the file storage area.
• **May vary dependent on whether the email message is HTML. For example, a 100-KB email message consumes 100 KB of data storage space. Text-only emails consume less storage than HTML due to only being the body text and not the html code and text version as well.
• ***Person Accounts consume roughly 4 KB. They are a junction of two individual records, an Account and a Contact. For example, 500,000 Person Accounts require around 2 GB of storage. Storage needed = (500000 X 4 KB)/1024 = 1953.125 MB which when further divided by 1024 = 1.9073 GB, which is around 2 GB.
• ****An order or Quote can have multiple line items related to them and this would not change the overall allocation of 2 KB for that record, this is due to the line items under this record are references from child to parent, and hence don't count towards the actual storage of the parent record.

See also
Data storage amount limit exceeded 
Calculate the storage size, which is provided by Salesforce