Pretty much as the tin says, assuming the Huns invade and conquer China and thus don’t move westward, how large could the Gothic state located in OTLs Romania and the Ukraine conceivably get?
It's unlikely that we would be talking of one Gothic state, or even two : Gutthiuda was, for exemple, an ensemble of Gothic and related peoples chiefdoms held together by a sense of political coalition and percieved kinship.
I'll prefer not mention the legendary dominion of Greuthingi and Ermanaric over the Pontic Steppe, just to say it's as credible as a 3$ bill : take it at face value at your own risk.
Gothic peoples (which would probably be seen as a larger ensemble between Pannonian Plain and Pontic Plain) would be dependent on their connection to Romania to prosper and form, from simple chiefdoms, complex or paramount chiefdoms and eventually early states. Now, even without Huns emerging as a paramount chiefdom in Europe (altough I find dubious at best that they would migrate estwards to China : I'd rather see them entering in Persia), a wave of Sarmatized eastern peoples would still go westwards under another name (probably part of groups that formed Huns in the IVth century) which would still push part of Goths in Romania. If ITTL, Romans manage to integrated smaller foedi in Moesia and Pannonia, it would allow already romanized Goths to get a presence on both banks of the Danube.
Basically, it would make Gothic chiefdoms having still access to a stronger Romania, with subsides, trade and mercenariate remaining tools of mobilisation and reinforcement of economical centralisation, without taking risk of forming a political threat to Romans using super-Gutthiude as an airbag against the likely other waves of steppe peoples.
I don't really see Gothic chiefdom holding more than northern Danubian basin and coast of Black Sea as such, altough who knows what eventually emerging early states could do, giving that the main center of political/economical/cultural gravity will be Rome and not underpopulated and backwards territories. At the very best, some sort of tributarisation of these areas and that's it, especially if a Gothic people manage to pull a paramount chiefdom out of this.
Not that it would be without consequences : A romanized Germano-Sarmatic enseble would have influence over Balto-Slavic formations.
Just a query in my own thread but over time is it possible for a Gothic homeland in the Ukraine to expand to include all of European Russia?
Not in the forseeable future : we're talking of complex chiefdoms at best, with all the unstability it implies. We couldn't speak of a Gothic empire there, as much the Hunnic empire was really a political/tributary network briefly maintained and dominated by Huns thanks to Roman subsides. Without these going out of hand in the Vth century, Gothic chiefdoms would have to make do, and probably barely able to expand their influence (and even more their territories) much northern, altough a strong influence over IOTL Ukraine is perfectly admittable.
The greatest challenge would be absorbing and assimilating the various Slavic groups.
I don't see it happening : not all Goths would be concerned by such network dominance, more likely Black Sea Goths, and even if they could pull it (which I don't think they could), they'd rather be slavicized as Huns were importantly germanized : the process would be all the more easy thanks to common sarmaticized elements.