I wish to withdraw my bitcoin to an external wallet address under my control. I have downloaded a wallet software (Bitcoin Core) but I read that it's a full node that validates and relays transactions on the bitcoin network and requires more space (over 145GB). Does it mean that its node cannot validate and relay transactions unless my computer has about 145GB space? How do I know if my computer has the required space? Thank you.
Yes, it will not work unless it is fully synced. This means the entire blockchain needs to be downloaded from the genesis block. Not only you that you need the required space but the sync can also take some time depending on the speed of your computer (RAM/CPU/HDD) and the connection speed (blocks need to be downloaded from other nodes).
Checking the available free disk space on your computer is dependent on your operating system, but it should be something very simple (most operating system show the available disk space near partition letters).
You could use pruning if you use a recent enough version of Bitcoin Core but this still requires download and verifying all blocks.
If you look for a solution that does not require disk space and syncs very fast, look for a SPV wallet like Electrum but be advised that SPV (Simplified Payment Verification) does not provide the same level of security as a full node. Use with caution and you should be safe.