Re: As a miner can i decide which block to mine?

Transactions can be up to 100kb in size (to be valid according to the network rules).

It's not. Transactions are valid as long as they can fit inside a single block.

No, thats not true.
Transactions are limited to a weight of 400.000. Look it up here:
Defined as constant here: https://github.com/bitcoin/bitcoin/blob/3c098a8aa0780009c11b66b1a5d488a928629ebf/src/policy/policy.h#L24.
And validty-check here: https://github.com/bitcoin/bitcoin/blob/3c098a8aa0780009c11b66b1a5d488a928629ebf/src/net_processing.cpp#L661-L665
Since the weight of a non-segwit transaction is 4 times its size, this leads to a maximum size of 100.000 byte (100kb) for valid transactions.

Note the comment:

Code:

// Ignore big transactions, to avoid a
// send-big-orphans memory exhaustion attack.

(to be found in 2nd link).