After winning an auction you will have to choose the storage size you want to associate to your domain. This will determine the maximum amount of data you can store on your domain.
The data stored in the Solana account can be words, images, IPFS CIDs, Arweaves hashes etc...
1kB is enough for non-advanced users. (Using .sol domains for transactions)
10kb is suggested for advanced users. (To resolve IPFS hosted website)
Cost for rent exemption is variable and is displayed live on the claim dialog. It is paid only once.
The average number of words is indicative only. On average, English text contains between 5 and 6.5 characters per word including spaces and punctuation.