Translation:the big bed
The bed is big = السرير كبير
Your sentence is actually a full sentence of a subject (السرير) and a predicative (كبير). Typically in Arabic, the predicative is not defined with AL.
In the sentence above we have two words, noun and adjective, and both are defined with AL. Thus, definitely, the second word here is not the predicative, but it must be an attributive adjective connected to the word السرير - meaning, it is an adjective that describes the bed already but not a predicative telling the status or some information about the bed. Attributive adjectives in Arabic mimic the noun they follow in definition, gender and number. For this reason, the above is just a phrase of noun and adjective, and not a full sentence (and there is no verb involved) so the English translation cannot involve (is) because, simply, we are not telling any information here. We are just pointing to some big bed; THE big bed.