Bob already mentioned it, it's 100kb or 100,000 bytes, it's displayed in the transaction's info if you click "Preview" in the "Send" Tab.

It seems like Electrum is trying to spend from too many inputs, just try to pick a few inputs with enough balance (about 30-60 inputs, refer on the steps below for the instructions on how to "coin control") from "coins" tab (View->Show Coins) and send it to the destination address.

Looks like you have a lot of small inputs in your wallet.
Try to consolidate your small inputs first:

• Open Coins tab.
• Select multiple inputs with small balance by holding "Shift" and Left click to highlight.
• Right click to the highlighted inputs and Select "Spend", you'll be transferred to the "Send" tab.
• From there, send to your own address and click "Max".... then, click "Preview", make sure the size is lesser than 100,000 bytes (100kb) in order to be valid.
• This method requires confirmation too, it will take a while for you to spend the output specially if you set a low fee.