You didn't explain why ypu want to reuse an address.
1. It is possible to re-use addresses, though there is rarely a good reason.
2. Unless I'm missing something, Electrum doesn't let you specify the change address.
3. The size of a transaction does not depend on which addresses are used.
3) YOu are wrong AFAIK...adresses which have a lot of inputs (like a public donation adress where people sent you a LOT of small transactions) actually ARE bigger in data size (Kb) and thus they might be more expensive to sent, since in bitcoin you pay sathoshie/per byte. That means the bigger the "adress" (the more inputs or outputs) it has, the biiger its data size is and thus the more you might pay to spent such adress... YOu didnt know this :-O...??
GUYS... wtf... i expect im talking to pros here...
Regarding why i wanna re-use one of my particular adresses is: I have written (that means backuped) the private key on paper on several places and im t lazy to replace all the backupfiles (written on paper) withthe new private key.
you don't have to use all unspent outputs... One address can be funded with hundreds of transactions, meaning the private key belonging to that address has to be used to create signatures for transaction spending any combination of those unspent outputs.
You don't have to create one big transaction spending all those unspent outputs at once...
So, it does not matter to the transaction size if you use one unspent output to address A or an unspent output to address B to fund address C.. It does matter how many unspent outputs you use....
Also a question, electrum wallet did generate by default 26 adresses (and corresponding private keys). If i sent small ammount of BTC "outside" (to an exchange for example). The "unspent" ammount will be automatically sent to ANY of the remaining 25 adresses it generated, correct? I by default cannot select the where the "unspent ammount", or the "change" will be sent, correct?
And what if i want to sent another (small) ammoutn outside, the "change" will be sent to another unused adress from the remaining 24 adresses electrum generated, correct? What if i sent additional 24 transcations, what then? Electrum will use one of those 26 adresses that it used before?
Electrum uses a gap limit... It automatically derives more addresses... It always uses the next unused address, it has a different derivation path for change addresses. So, no, when using electrum as a standard user, the change addresses will not be reused..